workflows: use bash shell explicitly

This forces better error handling as described in [1].

Without this change, bash would *not* run with `-o pipefail`, which
means some errors go unnoticed. By naming `bash` explicitly, `-o
pipefail` is enabled.

1:
https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#defaultsrunshell

.github/workflows/periodic-merge-24h.yml

@@ -16,6 +16,10 @@ on:
 
 permissions: {}
 
+defaults:
+  run:
+    shell: bash
+
 jobs:
   periodic-merge:
     if: github.repository_owner == 'NixOS'