ci/farm-rules: rename .disable-farm-mr-rules to make it clear it's only about MRs

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25243>

.gitlab-ci.yml

@@ -157,7 +157,7 @@ include:
 .build-rules:
   rules:
     # Don't run when disabling a farm
-    - !reference [.disable-farm-rules, rules]
+    - !reference [.disable-farm-mr-rules, rules]
     # If any files affecting the pipeline are changed, build/test jobs run
     # automatically once all dependency jobs have passed
     - changes: &all_paths
@@ -201,7 +201,7 @@ include:
 .container-rules:
   rules:
     # Don't run when disabling a farm
-    - !reference [.disable-farm-rules, rules]
+    - !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

.gitlab-ci/farm-rules.yml

@@ -239,11 +239,12 @@
 
 
 # Skip container & build jobs when disabling any farm
-.disable-farm-rules:
+# Only apply these rules in MR context, because otherwise we get a false
+# positive on files being 'created' when pushing to a new branch, and break
+# our pipeline
+.disable-farm-mr-rules:
   rules:
     # changes(disabled) + exists(disabled) = disabling the farm
-    # only run in MR context, because otherwise we get a false positive on
-    # files being 'created' when pushing to a branch, and nuke our pipeline
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
       changes: [ .ci-farms-disabled/microsoft ]
       exists: [ .ci-farms-disabled/microsoft ]