fix(ci): flutter workflow runs on go-ci runner, not ubuntu-latest

ubuntu-latest isn't a label any runner in this Forgejo deployment
advertises. The existing test.yml + release.yml both target the
project's go-ci label; flutter.yml should match. subosito/flutter-action@v2
installs the Flutter SDK on whatever runner picks up the job, so
go-ci is fine even though it isn't Flutter-pre-installed.

.forgejo/workflows/flutter.yml

@@ -27,7 +27,7 @@ on:
 
 jobs:
   analyze-test-build:
-    runs-on: ubuntu-latest
+    runs-on: go-ci
 
     defaults:
       run: