bump gh pages build to dev SDK (dart-archive/linter#4275)

Author: pq

.github/workflows/gh-pages.yml

@@ -21,7 +21,7 @@ jobs:
       - name: Setup Dart
         uses: dart-lang/setup-dart@d6a63dab3335f427404425de0fbfed4686d93c4f
         with:
-          sdk: stable
+          sdk: dev
       - name: Get dependencies
         run: dart pub get
       - name: Generate docs