ci: prevent Retest Workflow from running on forked repos

Forked repositories contain the the `.github/workflows/` directory, and
therefore run all the GitHub Workflows located there. Some of the
workflows need additional configuration, like providing access to the
standard `GITHUB_TOKEN`. If the extra configuration is not done, the
GitHub Workflow will fail, and the owner of the forked repository will
receive regular notifications about that.

There is no need to run the "retest" workflow on forked repositories, so
it can be skipped by default.

Signed-off-by: Niels de Vos <ndevos@ibm.com>

.github/workflows/retest.yaml

@@ -11,6 +11,7 @@ permissions:
 
 jobs:
   retest:
+    if: github.repository == 'ceph/ceph-csi'
     runs-on: ubuntu-latest
     steps:
       # path to the retest action