workflows/keep-sorted: make it fail

This never worked as intended, because it relies on "-o pipefail", which
is not set in GitHub Actions by default. It will be set when explicitly
selecting the shell as bash, though.

https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#exit-codes-and-error-action-preference

.github/workflows/keep-sorted.yml

@@ -36,5 +36,6 @@ jobs:
         run: "nix-env -f '<nixpkgs>' -iAP keep-sorted jq"
 
       - name: Check that Nix files are sorted
+        shell: bash
         run: |
           git ls-files | xargs keep-sorted --mode lint | jq --raw-output '.[] | "Please make sure any new entries in \(.path) are sorted alphabetically."'