workflows: lock Ubuntu runner to ubuntu-22.04

Lock the Ubuntu runner to ubuntu-22.04 to avoid accidental updates [1]
and increase reproducibility.

[1]: actions/runner-images#10636

(cherry picked from commit 2660dd1)

.github/workflows/periodic-merge-haskell-updates.yml

@@ -26,7 +26,7 @@ jobs:
       contents: write  # for devmasx/merge-branch to merge branches
       pull-requests: write  # for peter-evans/create-or-update-comment to create or update comment
     if: github.repository_owner == 'NixOS'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     name: git merge-base master staging → haskell-updates
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2