Use older ubuntu for pa11y

See pa11y/pa11y-ci#198

Author: Matt Henry

.github/workflows/test.yml

@@ -80,7 +80,7 @@ jobs:
         working-directory: ./backend
         run: npm run check-all
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       ENV: TESTING
       SAM_API_KEY: ${{ secrets.SAM_API_KEY }}"