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/test-source-dep.yml

@@ -200,6 +200,7 @@
 # Rules for unusual architectures that only build a subset of drivers
 .ppc64el-rules:
    rules:
+    - !reference [.never-post-merge-rules, rules]
     - !reference [.no_scheduled_pipelines-rules, rules]
     - !reference [.zink-common-rules, rules]
     - !reference [.softpipe-rules, rules]
@@ -212,6 +213,7 @@
 
 .s390x-rules:
    rules:
+    - !reference [.never-post-merge-rules, rules]
     - !reference [.no_scheduled_pipelines-rules, rules]
     - !reference [.zink-common-rules, rules]
     - !reference [.softpipe-rules, rules]
@@ -221,6 +223,7 @@
 # Rules for linters
 .lint-rustfmt-rules:
    rules:
+    - !reference [.never-post-merge-rules, rules]
     - !reference [.core-rules, rules]
     - changes:
       - src/**/*.rs
@@ -228,6 +231,7 @@
 
 .lint-clang-format-rules:
    rules:
+    - !reference [.never-post-merge-rules, rules]
     - !reference [.core-rules, rules]
     - changes:
       - .clang-format