CI: Skip OpenTitan build job on forks

Port of mlkem-native#1649.

Signed-off-by: Matthias J. Kannwischer <matthias@zerorisc.com>

Author: mkannwischer

.github/workflows/integration-opentitan.yml

@@ -54,6 +54,7 @@ jobs:
   opentitan_build:
     name: OpenTitan ML-DSA Build Test
     needs: start-ec2-runner
+    if: ${{ github.repository_owner == 'pq-code-package' && !github.event.pull_request.head.repo.fork }}
     runs-on: ${{ needs.start-ec2-runner.outputs.label }}
     steps:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0