build: use PR number for title check concurrency (#197)

The github.ref may just be the target branch name, which results in all
PR title checks sharing the same concurrency group. This change uses the
PR number (if available) as the discriminator.

Signed-off-by: Mark S. Lewis <Mark.S.Lewis@outlook.com>

Author: bestbeforetoday

.github/workflows/pr_title.yml

@@ -6,7 +6,7 @@ on:
 
 # Most recent PR change supersedes previous changes.
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
   cancel-in-progress: true
 
 # Default to the minimum read-only token for all jobs.