test: Fix external PR trigger
test: Fix external PR trigger

External PRs should not run on our testing infrastructure automatically.
They can be verified by an admin with the 'Test this please.' phrase.

coreos/prometheus-operator,brancz/prometheus-operator,brancz/prometheus-operator,eedugon/prometheus-operator,coreos/prometheus-operator,mmerrill3/prometheus-operator,gianrubio/prometheus-operator,gianrubio/prometheus-operator,jescarri/prometheus-operator,jescarri/prometheus-operator,gianrubio/prometheus-operator,brancz/prometheus-operator,eedugon/prometheus-operator,mmerrill3/prometheus-operator,mmerrill3/prometheus-operator