WIP: downgrade ubuntu version to fix CI

There have been errors in CI about ChromeHeadless. I suspect this isn’t
due to my changes, but is instead because `ubuntu-latest` has recently
changed from 22.04 to 24.04. This commit downgrades CI back to ubuntu
22.04, which will hopefully fix the issue.

On [run
574](https://github.com/guardian/facia-tool/actions/runs/11294105168/job/31413785727),
which was the last successful CI run, the version used was still 22.04,
which is why I suspect this is the culprit.

.github/workflows/improvedci.yml

@@ -60,7 +60,7 @@ jobs:
                   path: public/src/jspm_packages
                   if-no-files-found: error
     test_client_v1:
-        runs-on: ubuntu-latest
+        runs-on: ubuntu-22.04
         permissions:
             id-token: write
             contents: read