Validate PRs against correct branch (#5344)

(cherry picked from commit 720598c)

Author: pquentin

.github/workflows/validate-pr.yml

@@ -26,7 +26,7 @@ jobs:
           repository: elastic/clients-flight-recorder
           path: ./clients-flight-recorder
           token: ${{ secrets.PAT }}
-          ref: main
+          ref: ${{ github.base_ref }}
 
       - name: Use Node.js 22
         uses: actions/setup-node@v4