Fix check for PRs from forks.

Solution from https://github.community/t/how-to-detect-a-pull-request-from-a-fork/18363/3.

Author: bgrainger

.github/workflows/sonarcloud.yml

@@ -14,7 +14,7 @@ jobs:
   build:
     name: Scan
     runs-on: windows-latest
-    if: github.repository_owner == 'mysql-net'
+    if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository
     steps:
       - name: Set up JDK 11
         uses: actions/setup-java@v1