ci/gpu: do not update ref on PR

(cherry picked from commit e7fc2d8)

.azure/gpu-unittests.yml

@@ -181,6 +181,8 @@ jobs:
           # copy potentially updated cache to the machine filesystem to be reused with next jobs
           cp -r --update tests/_cache-references /var/tmp/cached-references
         # set as extra step to not pollute general cache when jobs fails or crashes
+        # so do this update only with successful jobs on master
+        condition: and(succeeded(), ne(variables['Build.Reason'], 'PullRequest'))
         displayName: "Update cached refs"
 
       - bash: |