chore: exclude build/ from dart analyzer

Prevents analyzer from scanning vendored Firebase sources in
build/ios/SourcePackages/, which aren't part of the project.

Author: jakobkoerber

analysis_options.yaml

@@ -9,6 +9,7 @@ analyzer:
   errors:
     invalid_annotation_target: ignore
   exclude:
+    - 'build/**'
     - '**/*.g.dart'
     - lib/base/networking/apis/tumdev/campus_backend.pb.dart
     - lib/base/networking/apis/tumdev/campus_backend.pbenum.dart