fix(ci): Fix macos and linux CI by using a supported image

The github action was failing to execute because github no longer
maintains any action runners with the OS versions that were being
requested.

Instead just use the latest, so CI should continue to work and not be
dependent on the end of life of certain OS versions.

.github/workflows/main.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-18.04, macos-11, windows-2019]
+        os: [ubuntu-latest, macos-latest, windows-latest]
 
     steps:
       - uses: actions/checkout@v2
@@ -46,7 +46,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-18.04, macos-10.15]
+        os: [ubuntu-latest, macos-latest]
 
     steps:
       - uses: actions/checkout@v2
@@ -63,5 +63,5 @@ jobs:
 
       - name: Build
         run: |
-          bazelisk build  --platforms=@local_config_platform//:host //... 
+          bazelisk build  --platforms=@local_config_platform//:host //...