test in ubuntu 20.04

.github/workflows/tests.yml

@@ -26,7 +26,7 @@ jobs:
   test-job:
     strategy:
       matrix:
-        os: [ubuntu-20.04, windows-2019, macos-12]
+        os: [ubuntu-20.04, ubuntu-22.04, windows-2019, macos-12]
         # Python 3.8 is what we currently support for running cohortextractor
         # locally, and 3.9 is what we required for databuilder so we need to make
         # sure we can run with those
@@ -39,6 +39,9 @@ jobs:
         with:
           persist-credentials: false
 
+      - name: Ensure pip cache dir exists
+        run: mkdir -p ~/.cache/pip
+
       - name: Set up Python ${{ matrix.python }}
         uses: actions/setup-python@v5
         with: