Only run self-service compat job on PRs (#11893)

This is currently failing for the notice file update PR. While we
could try to hack around the git commands to do something sensible, I
don’t really see the point. We really only need this for PRs.

changelog_begin
changelog_end

ci/prs.yml

@@ -174,6 +174,7 @@ jobs:
 - job: self_service_compat_test
   pool:
     vmImage: ubuntu-20.04
+  condition: eq(variables['Build.Reason'], 'PullRequest')
   steps:
   - checkout: self
   - template: bash-lib.yml