Run the Github verification with GTK4 enabled if it has GTK4 label

Author: laeubi

.github/workflows/maven.yml

@@ -12,6 +12,9 @@ on:
   pull_request:
     branches: [ master ]
 
+  env:
+    SWT_GTK4: "${{ contains(github.event.pull_request.labels.*.name, 'gtk4') && '1' || '0' }}"
+
 jobs:
   event_file:
     name: "Event File"