workflows: prevent accidental cancelling of other PRs

This can happen when two PRs run at the same time, which come from
different forks, but have the same head branch name.

github.head_ref is suggested by GitHub's docs, but.. that's not really
useful for cases with forks.

(cherry picked from commit 7ba7720b28)

.github/workflows/check.yml

@@ -7,7 +7,7 @@ on:
   pull_request_target:
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.event_name }}-${{ github.head_ref || github.run_id }}
+  group: ${{ github.workflow }}-${{ github.event_name }}-${{ github.event.pull_request.number || github.run_id }}
   cancel-in-progress: true
 
 permissions: {}