build: disable SonarCloud for fork pull requests (#4142)

The `SonarCloud Build` workflow currently runs on all pull requests,
including those originating from forks. However, SonarCloud requires
secrets to publish its analysis, and these secrets are (by design) not
exposed to pull requests from forks.

This causes the workflow to consistently fail for any external
contributor or branch not residing on the main repository, creating
noise in the PR checks.

This PR adds a standard GitHub Actions `if` condition to the `build` job
to simply skip the workflow if the pull request originates from a fork.

Author: westarle

.github/workflows/sonar.yaml

@@ -8,6 +8,7 @@ on:
 jobs:
   build:
     name: Build
+    if: github.event.pull_request.head.repo.full_name == github.repository || github.event_name != 'pull_request'
     runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v3