PR from forked repo fix (#105)

Author: MarekMaj

.github/workflows/build.yml

@@ -48,4 +48,4 @@ jobs:
           token: ${{ secrets.GITHUB_TOKEN }}
           min-coverage-overall: 40
           min-coverage-changed-files: 60
-        if: startsWith(matrix.flink, '1.18')
+        if: startsWith(matrix.flink, '1.18') && github.event.pull_request.head.repo.fork == false