github: prevent parallel runs of gh-pages update

Ensure sequential updates to the gh-pages branch. Otherwise, if parallel
jobs are triggered, onnly one (the first to complete) will succeed and
the others will fail with git push error (because the tip of the
gh-pages has moved and their commit doesn't apply anymore).

.github/workflows/gh-pages.yml

@@ -8,6 +8,10 @@ on:
       - v[0-9]+.[0-9]+.[0-9]+
   release:
     types: [published]
+
+concurrency:
+  group: ${{ github.workflow }}
+
 jobs:
   build:
     name: Update gh-pages