Prevent PR builds from running on forks

Author: elliedori

.github/workflows/pr-build-workflow.yml

@@ -6,7 +6,7 @@ jobs:
   build:
     name: Build
     runs-on: ubuntu-latest
-
+    if: github.repository == 'spring-projects/spring-security'
     steps:
       - uses: actions/checkout@v2
       - name: Set up JDK