fix: prevent duplicate release PRs with concurrency group (#397)

Add concurrency control to release-plz workflow to prevent race condition
where merging a release PR triggers another workflow run that creates a
duplicate release PR before the first run completes publishing.

The race happens because:
1. Merge release PR -> triggers release-plz run
2. Main branch update -> triggers another release-plz run
3. Second run checks state before first completes
4. Creates duplicate PR for same version

Solution: Only allow one release-plz run at a time with cancel-in-progress: false
to ensure the first run completes before the next one starts.

Author: joshrotenberg

.github/workflows/release-plz.yml

@@ -9,6 +9,10 @@ on:
     branches:
       - main
 
+concurrency:
+  group: release-plz
+  cancel-in-progress: false
+
 jobs:
   release-plz:
     name: Release-plz