Fail explicitly with pull requests opened from forks (#2580)

Author: pquentin

.github/workflows/validate-pr.yml

@@ -15,10 +15,13 @@ on:
 jobs:
   validate-pr:
     name: build
-    if: github.repository_owner == 'elastic' # this action will fail if executed from a fork
     runs-on: ubuntu-latest
 
     steps:
+      - name: Check pull request was opened from branch
+        if: github.event.pull_request.head.repo.full_name != 'elastic/elasticsearch-specification'
+        run: echo "Validation is not supported from forks"; exit 1
+        
       - uses: actions/checkout@v4
         with:
           path: ./elasticsearch-specification