Lock the GitHub CI runner to a specific Ubuntu version

We need to have control over our CI environment - we lock Fedora to a
certain version without updates, yet the underlying Ubuntu image has
been floating and causing trouble every now and then. Like just now,
with ubuntu-latest moving from 22.04 to 24.04 where sudo is somehow
broken inside our test-suite and causing totally unrelated PR's to
fail with

   sudo: PAM account management error: Authentication service cannot retrieve authentication info
   sudo: a password is required

Lock the version to Ubuntu 22.04 for now, we'll need to eventually
figure what the issue is but these things need to happen on our terms.

.github/workflows/linux.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   build_and_test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
 
     steps:
       - uses: actions/checkout@v3