fix: cloud build dart yaml is done before the /app image (#4956)

tested locally with the following to see the image get built

```shell
podman run --rm \
  --platform linux/amd64 \
  --entrypoint /bin/bash \
  -v $(pwd):/workspace \
  -w /workspace \
  us-docker.pkg.dev/flutter-dashboard/flutter/flutter \
  -c "cloud_build/dashboard_build.sh"

podman build \
  --platform linux/amd64 \
  -t us-docker.pkg.dev/flutter-dashboard/appengine/default.version-manual-build \
  -f Dockerfile.app_dart .
```

inspecting the image:
```shell
# pwd
/app
# ls
analyze  app_dart  auto_submit	dashboard  dev	licenses  packages  pubspec.lock  pubspec.yaml
# ls app_dart/build
native_assets  web
# ls app_dart/build/web
assets	canvaskit  favicon-failure.png	favicon.png  flutter.js  flutter_bootstrap.js  flutter_service_worker.js  icons  index.html  main.dart.js  main.dart.js.map  manifest.json  version.json
```

Author: jtmcdole

cloud_build/dashboard_build.sh

@@ -5,7 +5,7 @@
 
 # Build flutter build dashboard.
 
-pushd app/dashboard > /dev/null
+pushd dashboard > /dev/null
 set -e
 rm -rf build
 flutter doctor -v