Run CI builds in Ubuntu 18.04 container to use Python 2

Latest Ubuntu versions don't provide it any more.

.github/workflows/ci.yml

@@ -6,15 +6,17 @@ on: [ push, pull_request ]
 jobs:
  build:
  runs-on: ubuntu-latest
+ # We need to use an OLS old enough to still have Python 2
+ container: ubuntu:18.04
 
  steps:
- - name: Check out
- uses: actions/checkout@v2
+ - name: Set up
+ run: |
+ apt-get update -qq
+ apt-get install -qq git make python2
 
- - name: Set up Python 2.7
- uses: actions/setup-python@v2
- with:
- python-version: 2.7
+ - name: Check out
+ uses: actions/checkout@v3
 
  - name: Install dependencies
  run: |