ci/windows: Add a variable to globally disable jobs using windows runners

Suggested-by: Daniel Stone <daniels@collabora.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16055>

.gitlab-ci.yml

@@ -19,6 +19,7 @@ variables:
   # running on a particular CI farm (ie. for outages, etc):
   FD_FARM: "online"
   COLLABORA_FARM: "online"
+  MICROSOFT_FARM: "online"
 
 default:
   before_script:

.gitlab-ci/test-source-dep.yml

@@ -587,6 +587,8 @@
 # rules duplication manually
 .windows-build-rules:
   rules:
+    - if: '$MICROSOFT_FARM == "offline"'
+      when: never
     - *ignore_scheduled_pipelines
     - changes: 
         *mesa_core_file_list
@@ -621,6 +623,8 @@
 
 .windows-test-rules:
   rules:
+    - if: '$MICROSOFT_FARM == "offline"'
+      when: never
     - *ignore_scheduled_pipelines
     - changes:
         *mesa_core_file_list