[CI] Don't use runner.name as a key for ubuntu's ccache (#1878)

`${{runner.name}}` refers to

> The name of the runner executing the job. This name may not be unique
in a workflow run as runners at the repository and organization levels
could use the same name.


[source](https://docs.github.com/en/actions/learn-github-actions/contexts#runner-context)

It may be too unique since after this change, ccache never finds cached
data (hit rate ~ 0%). This defeats the purpose of using a cache.

Using a fixed value may lead to a higher hit rate.

.github/workflows/ubuntu.yml

@@ -30,7 +30,7 @@ jobs:
     name: Build
     env:
       ORTOOLS_DIR: ${{ github.workspace }}/or-tools
-      OS: ubuntu-20.04
+      os: ubuntu-20.04
 
     runs-on: ubuntu-20.04
     if: "!contains(github.event.head_commit.message, '[skip ci]')"
@@ -41,7 +41,7 @@ jobs:
     - name: ccache
       uses: hendrikmuhs/[email protected]
       with:
-          key: ${{ runner.name }}
+          key: ${{ env.os }}
 
     - name: Install libraries
       run: |
@@ -66,7 +66,7 @@ jobs:
       uses: ./.github/workflows/download-extract-precompiled-libraries-tgz
       with:
         antares-deps-version: ${{steps.antares-deps-version.outputs.prop}}
-        os: ${{env.OS}}
+        os: ${{env.os}}
         ortools-url: ${{env.ORTOOLS_URL}}
         ortools-dir: ${{env.ORTOOLS_DIR}}
 
@@ -121,7 +121,7 @@ jobs:
       with:
         simtest-tag: ${{steps.simtest-version.outputs.prop}}
         batch-name: valid-named-mps
-        os: ${{ env.OS }}
+        os: ${{ env.os }}
         variant: "named-mps"
 
     - name: Run unfeasibility-related tests
@@ -149,7 +149,7 @@ jobs:
       with:
         simtest-tag: ${{steps.simtest-version.outputs.prop}}
         batch-name: valid-v830
-        os: ${{ env.OS }}
+        os: ${{ env.os }}
 
     - name: Run MILP with CBC
       if: ${{ env.RUN_SIMPLE_TESTS == 'true' }}
@@ -158,55 +158,55 @@ jobs:
         simtest-tag: ${{steps.simtest-version.outputs.prop}}
         batch-name: valid-milp
         variant: "milp-cbc"
-        os: ${{ env.OS }}
+        os: ${{ env.os }}
 
     - name: Run tests introduced in v860
       if: ${{ env.RUN_SIMPLE_TESTS == 'true' }}
       uses: ./.github/workflows/run-tests
       with:
         simtest-tag: ${{steps.simtest-version.outputs.prop}}
         batch-name: valid-v860
-        os: ${{ env.OS }}
+        os: ${{ env.os }}
 
     - name: Run tests introduced in v870
       if: ${{ env.RUN_SIMPLE_TESTS == 'true' }}
       uses: ./.github/workflows/run-tests
       with:
         simtest-tag: ${{steps.simtest-version.outputs.prop}}
         batch-name: valid-v870
-        os: ${{ env.OS }}
+        os: ${{ env.os }}
 
     - name: Run short-tests
       if: ${{ env.RUN_SIMPLE_TESTS == 'true' }}
       uses: ./.github/workflows/run-tests
       with:
         simtest-tag: ${{steps.simtest-version.outputs.prop}}
         batch-name: short-tests
-        os: ${{ env.OS }}
+        os: ${{ env.os }}
 
     - name: Run mps tests
       if: ${{ env.RUN_SIMPLE_TESTS == 'true' }}
       uses: ./.github/workflows/run-tests
       with:
         simtest-tag: ${{steps.simtest-version.outputs.prop}}
         batch-name: valid-mps
-        os: ${{ env.OS }}
+        os: ${{ env.os }}
 
     - name: Run tests for adequacy patch (CSR)
       if: ${{ env.RUN_SIMPLE_TESTS == 'true' }}
       uses: ./.github/workflows/run-tests
       with:
         simtest-tag: ${{steps.simtest-version.outputs.prop}}
         batch-name: adequacy-patch-CSR
-        os: ${{ env.OS }}
+        os: ${{ env.os }}
 
     - name: Run parallel tests
       if: ${{ env.RUN_EXTENDED_TESTS == 'true' }}
       uses: ./.github/workflows/run-tests
       with:
         simtest-tag: ${{steps.simtest-version.outputs.prop}}
         batch-name: valid-parallel
-        os: ${{ env.OS }}
+        os: ${{ env.os }}
         variant: "parallel"
 
     - name: Run medium-tests
@@ -215,31 +215,31 @@ jobs:
       with:
         simtest-tag: ${{steps.simtest-version.outputs.prop}}
         batch-name: medium-tests
-        os: ${{ env.OS }}
+        os: ${{ env.os }}
 
     - name: Run long-tests-1
       if: ${{ env.RUN_EXTENDED_TESTS == 'true' }}
       uses: ./.github/workflows/run-tests
       with:
         simtest-tag: ${{steps.simtest-version.outputs.prop}}
         batch-name: long-tests-1
-        os: ${{ env.OS }}
+        os: ${{ env.os }}
 
     - name: Run long-tests-2
       if: ${{ env.RUN_EXTENDED_TESTS == 'true' }}
       uses: ./.github/workflows/run-tests
       with:
         simtest-tag: ${{steps.simtest-version.outputs.prop}}
         batch-name: long-tests-2
-        os: ${{ env.OS }}
+        os: ${{ env.os }}
 
     - name: Run long-tests-3
       if: ${{ env.RUN_EXTENDED_TESTS == 'true' }}
       uses: ./.github/workflows/run-tests
       with:
         simtest-tag: ${{steps.simtest-version.outputs.prop}}
         batch-name: long-tests-3
-        os: ${{ env.OS }}
+        os: ${{ env.os }}
 
     - name: Installer .deb creation
       run: |

.github/workflows/windows-vcpkg.yml

@@ -33,7 +33,7 @@ jobs:
       VCPKG_ROOT: ${{ github.workspace }}/vcpkg
       ORTOOLS_DIR: ${{ github.workspace }}/or-tools
       RUN_EXTENDED_TESTS: ${{ github.event_name == 'schedule'}}
-      OS: windows-latest
+      os: windows-latest
       test-platform: windows-2022
       vcpkgPackages: wxwidgets boost-test
       triplet: x64-windows
@@ -111,7 +111,7 @@ jobs:
       uses: ./.github/workflows/download-extract-precompiled-libraries-zip
       with:
         antares-deps-version: ${{steps.antares-deps-version.outputs.prop}}
-        os: ${{env.OS}}
+        os: ${{env.os}}
         ortools-url: ${{env.ORTOOLS_URL}}
         ortools-dir: ${{env.ORTOOLS_DIR}}