fix: refine concurrency policy of `check-urls.yml` workflow (#7096)

* fix: refine `check-urls.yml` workflow concurrency policy

This parametrization makes don't fail with a cancellation

https://github.com/EbookFoundation/free-programming-books/commit/25a05812f7ed2399352b1c9fbacb1226b8ffac69#comments

* typo: concurrency code comment

.github/workflows/check-urls.yml

@@ -7,10 +7,10 @@ on:
 permissions:
   contents: read
 
-# This allows a subsequently queued workflow run to interrupt previous runs
+# This allows a subsequently queued workflow run to interrupt/wait for previous runs
 concurrency:
-  group: '${{ github.workflow }} @ ${{ github.event.pull_request.head.label || github.head_ref || github.ref || github.run_id }}'
-  cancel-in-progress: true
+  group: '${{ github.workflow }} @ ${{ github.run_id }}'
+  cancel-in-progress: false         # true = interrupt, false = wait
 
 jobs:
   check-urls: