trigger compat tests _before_ merging to master

.gitlab-ci.yml

@@ -38,6 +38,7 @@ variables:
 .global_trigger_compat_tests: &global_trigger_compat_tests
   rules:
     - if: $CI_COMMIT_BRANCH == "master"
+    - if: $CI_EXTERNAL_PULL_REQUEST_TARGET_BRANCH_NAME == "master"
 
 #
 # Test environment & platform settings