Avoid submitting a dependency graph for forks

Since the "dependency graph" feature needs to be enabled per repository,
running the workflow on forks caused it to fail.

.github/workflows/gradle-dependency-submission.yml

@@ -10,6 +10,7 @@ permissions:
 
 jobs:
   dependency-submission:
+    if: github.repository == 'junit-team/junit5'
     runs-on: ubuntu-latest
     steps:
     - name: Check out repository