Pin Ubuntu to 22.04 for Python 3.7 workflow

Fix the following error at the setup-python step:

```
Version 3.7 was not found in the local cache
Error: The version '3.7' with architecture 'x64' was not found for Ubuntu 24.04.
The list of all available versions can be found here: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json
```

See e.g. https://github.com/galaxyproject/galaxy/actions/runs/11252595341/job/31294713655

.github/workflows/test_galaxy_packages_for_pulsar.yaml

@@ -16,7 +16,7 @@ concurrency:
 jobs:
   test:
     name: Test
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04  # Python 3.7 is not available via setup-python on ubuntu >=24.04
     strategy:
       fail-fast: false
       matrix: