Make ci self sign test only run on non-forks

Signed-off-by: Appu <appu@google.com>

.github/workflows/ci.yaml

@@ -61,6 +61,7 @@ jobs:
       run: ./gradlew build
 
     - name: Ensure sigstore-java self signing still works
+      if: ${{ !github.event.pull_request.head.repo.fork }}
       run: ./gradlew sigstore-java:publishToMavenLocal -Prelease -PskipPgpSigning
 
     - name: Test sigstore-java/sandbox