Always use stable sdk for CI runs

Some of the newer linter features require a more recent SDK. Forcing the
use of the "stable" SDK for CI runs should ensure that we don't
accidentally miss a lint warning that pub catches after a new version is
published.

.github/workflows/ci.yaml

@@ -12,6 +12,8 @@ jobs:
 
       - name: Setup Dart SDK
         uses: dart-lang/setup-dart@v1
+        with:
+          sdk: stable
 
       - name: Prepare RabbitMQ configuration
         run: |