ci: Do not run style check on stable branches

The style check job currently fails on stable branches since the recent
uncrustify change. We could use older image on stable, or backport the code
changes, but I think that the style check job on stable branches is not
actually needed. Let's disable the style check job on stable branches to
fix pipeline there and avoid this situation in the future.

.gitlab-ci.yml

@@ -67,6 +67,8 @@ style check:
         - gcc data/lineup-parameters.c `pkg-config --cflags --libs gio-2.0 gio-unix-2.0` -o data/lineup-parameters
         - LANG=C.utf8 data/run-uncrustify.sh
         - git diff --exit-code | tee uncrustify.diff
+    except:
+        - /^gnome-.*$/
 
 update image:
     variables: