Install Flutter dependencies even for formatting via Dart

Dart will automatically check and utilize any of the defined
analysis_options.yaml configuration settings, and since we use the
conventional `package:flutter_lints/flutter.yaml`, if we haven't
installed our dependencies, the output will loudly complain about
failing to resolve the package.

Author: elasticdog

.github/workflows/check-flutter.yml

@@ -16,6 +16,8 @@ jobs:
           channel: stable
           flutter-version-file: pubspec.yaml
           cache: true
+      - name: Install dependencies
+        run: flutter pub get
       - name: Check formatting
         run: dart format --output=none --set-exit-if-changed .