ci: Prevent pages job from running in pre-merge pipelines

Since pre-merge pipelines run in the "mesa" namespace now, the rule
would have created the job if the source branch was named "master" (and
the job might have updated the public website).

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6566>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 076e64e..15eb5c8 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -54,7 +54,7 @@
     - if: &is-forked-branch-or-pre-merge '$CI_PROJECT_NAMESPACE != "mesa" || $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME == $CI_COMMIT_REF_NAME'
       when: manual
     # Pipeline runs for the master branch of the main project
-    - if: &is-main-master '$CI_PROJECT_NAMESPACE == "mesa" && $CI_COMMIT_REF_NAME == "master"'
+    - if: &is-main-master '$CI_PROJECT_NAMESPACE == "mesa" && $CI_COMMIT_REF_NAME == "master" && $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME != "master"'
       when: always
     # Post-merge pipeline
     - if: &is-post-merge '$CI_PROJECT_NAMESPACE == "mesa" && $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME != $CI_COMMIT_REF_NAME'