Add concurrency control to PR validation workflow

Author: devcrocod

.github/workflows/validate-pr.yml

@@ -5,6 +5,10 @@ on:
   pull_request:
     branches: [main]
 
+concurrency:
+    group: ${{ github.workflow }}-${{ github.ref }}
+    cancel-in-progress: true
+
 jobs:
   validate-pr:
     runs-on: macos-latest