chore: use sha instead of ref

Author: Ang-dot

.github/workflows/validate-new-plugin-metadata.yml

@@ -16,7 +16,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           fetch-depth: 0
-          ref: ${{ github.event.pull_request.head.ref }}
+          ref: ${{ github.event.pull_request.head.sha }}
 
       - name: Identify New Plugin Directories
         id: find_new_plugins