ci: disable gtk4 build by default

It's not used by anything in the monorepo, only used when
rebuilding the gstreamer-rs image (used by gst-plugins-rs
where the gtk4 plugin lives).

Doesn't make sense to build all of gtk4 on every CI run that
touches the relevant files just to make sure it still works
for the image rebuild, and it's a problem for users with long
filenames (such as `gstreamer-backport-bot`) where the pipeline
will simply not pass because the gtk glib-mkenums command line
length exceeds 32767 characters.

Better to set up a scheduled pipeline for that.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8636>

.gitlab-ci.yml

@@ -73,6 +73,9 @@ variables:
 
   FF_TIMESTAMPS: true
 
+  # Can set this as part of a schedule pipeline to test the gtk4 build if desired
+  # GTK_ARGS: "-Dgtk=enabled"
+
 workflow:
   # https://docs.gitlab.com/ee/ci/yaml/index.html#switch-between-branch-pipelines-and-merge-request-pipelines
   rules:
@@ -330,17 +333,6 @@ pre-commit checks:
     - if: '$CI_PROJECT_NAMESPACE == "gstreamer" && $CI_COMMIT_BRANCH == $GST_UPSTREAM_BRANCH'
       when: manual
 
-# Check whether the gtk wrap had any changes, and if so, build it
-.gtk-build-rules:
-  rules:
-    - changes:
-        paths:
-          - ".gitlab-ci.yml"
-          - "meson.build"
-          - "subprojects/*.wrap"
-      variables:
-        GTK_ARGS: "-Dgtk=enabled"
-
 .build:
   stage: 'build'
   needs:
@@ -509,7 +501,6 @@ build debian x86_64:
       -Dgst-plugins-good:soup=enabled
       -Dgst-plugins-bad:d3d11-wgc=enabled
   rules:
-    - !reference [.gtk-build-rules, rules]
     - !reference [.upstream-branch-rules, rules]
     - changes:
         paths:
@@ -635,7 +626,6 @@ build macos:
     MESON_ARGS: "${DEFAULT_MESON_ARGS}"
     SUBPROJECTS_CACHE_DIR: "/Users/gst-ci/subprojects"
   rules:
-    - !reference [.gtk-build-rules, rules]
     - !reference [.upstream-branch-rules, rules]
     - changes:
         paths: