CI: cancel workflows when PRs are updated (#16562)

For checkstyle, zloop, zfs-qemu, and codeql workflows cancel
in-progress jobs when the PR is updated.

Relevant GitHub Actions documentation:

  The following concurrency group cancels in-progress jobs or run
  on pull_request events only; if github.head_ref is undefined, the
  concurrency group will fallback to the run ID, which is guaranteed
  to be both unique and defined for the run.

https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#example-using-a-fallback-value

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #16562

.github/workflows/zloop.yml

@@ -4,6 +4,10 @@ on:
   push:
   pull_request:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 jobs:
   zloop:
     runs-on: ubuntu-24.04