ci: drop containers, builds, and tests from post-merge pipeline

All these jobs are redundant and a waste of resources:
- the containers have already been built & pushed in the merge pipeline
- the mesa build variants have already all passed
- the driver tests have already all passed

None of these jobs are doing anything useful in this pipeline, but it
costs a factor of 2x to our infrastructure, so let's remove them.

In other words, the only job left in the post-merge pipeline is the
`pages` job that deploys the update to the website.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26451>

.gitlab-ci.yml

@@ -32,11 +32,7 @@ workflow:
         MESA_CI_PERFORMANCE_ENABLED: 1
         VALVE_INFRA_VANGOGH_JOB_PRIORITY: ""  # Empty tags are ignored by gitlab
     # post-merge pipeline
-    - if: $GITLAB_USER_LOGIN == "marge-bot" && $CI_COMMIT_BRANCH
-      variables:
-        KERNEL_IMAGE_BASE: https://${S3_HOST}/mesa-lava/${KERNEL_REPO}/${KERNEL_TAG}
-        JOB_PRIORITY: 40
-        VALVE_INFRA_VANGOGH_JOB_PRIORITY: priority:low
+    - if: &is-post-merge $GITLAB_USER_LOGIN == "marge-bot" && $CI_PIPELINE_SOURCE == "push"
     # any other pipeline
     - if: $GITLAB_USER_LOGIN != "marge-bot" && $FORCE_KERNEL_TAG != null
       variables:
@@ -168,19 +164,13 @@ include:
 # YAML anchors for rule conditions
 # --------------------------------
 .rules-anchors:
-  # Post-merge pipeline
-  - &is-post-merge '$CI_PROJECT_NAMESPACE == "mesa" && $CI_COMMIT_BRANCH'
-  # Post-merge pipeline after Marge has merged an MR
-  - &is-post-merge-for-marge '$CI_PROJECT_NAMESPACE == "mesa" && $CI_COMMIT_BRANCH && $GITLAB_USER_LOGIN != "marge-bot"'
-  # Post-merge pipeline, not for Marge Bot
-  - &is-post-merge-not-for-marge '$CI_PROJECT_NAMESPACE == "mesa" && $GITLAB_USER_LOGIN != "marge-bot" && $CI_COMMIT_BRANCH'
   # Pre-merge pipeline
   - &is-pre-merge '$CI_PIPELINE_SOURCE == "merge_request_event"'
 
 
 .never-post-merge-rules:
   rules:
-    - if: *is-post-merge-for-marge
+    - if: *is-post-merge
       when: never
 
 
@@ -188,9 +178,11 @@ include:
   rules:
     # Run when re-enabling a disabled farm, but not when disabling it
     - !reference [.disable-farm-mr-rules, rules]
-    # Run pipeline by default in the main project if any CI pipeline
-    # configuration files were changed, to ensure docker images are up to date
-    - if: *is-post-merge
+    # Never run immediately after merging, as we just ran everything
+    - !reference [.never-post-merge-rules, rules]
+    # Run pipeline by default if it was triggered by Marge Bot, is for a
+    # merge request, and any files affecting the pipeline were changed
+    - if: *is-merge-attempt
       changes: &all_paths
         - VERSION
         - bin/git_sha1_gen.py
@@ -211,24 +203,10 @@ include:
         - include/**/*
         - src/**/*
       when: on_success
-    # Run pipeline by default if it was triggered by Marge Bot, is for a
-    # merge request, and any files affecting the pipeline were changed
-    - if: *is-merge-attempt
-      changes:
-        *all_paths
-      when: on_success
-    # Run pipeline by default in the main project if it was not triggered by
-    # Marge Bot, and any files affecting the pipeline were changed
-    - if: *is-post-merge-not-for-marge
-      changes:
-        *all_paths
-      when: on_success
     # Just skip everything for MRs which don't actually change anything in the
     # build - the same rules as above, but without the file-change rules
     - if: *is-merge-attempt
       when: never
-    - if: *is-post-merge
-      when: never
     # Always allow user branches etc to trigger jobs manually
     - when: manual