ci/contrib: use separate concurrency group

Run the contrib workflow in its own concurrency group to avoid multiple
runs causing conflicting git pushes.

The contrib workflow pushes to the gh-pages branch, and thus is not safe
to be run multiple times in parallel. Given that github-actions can
stall workflow runs for quite some time, multiple executions can
overtake each other. By using concurrency groups, all workflow runs will
wait until previous runs completed.

We explicitly disable cancellation to get better diagnostics and more
easily find the first possible run that failed. Since these workflow
runs are rather fast, and only run on master, cancellation seems not
necessary.

Author: dvdhrm

.github/workflows/contrib.yml

@@ -4,6 +4,10 @@ on:
     branches:
       - master
 
+concurrency:
+  cancel-in-progress: false
+  group: "gh-pages"
+
 permissions:
   contents: read