Bump dart-lang/setup-dart from 1.3 to 1.4 (dart-archive/appengine#158)

Bumps [dart-lang/setup-dart](https://github.com/dart-lang/setup-dart) from 1.3 to 1.4.
- [Release notes](https://github.com/dart-lang/setup-dart/releases)
- [Changelog](https://github.com/dart-lang/setup-dart/blob/main/CHANGELOG.md)
- [Commits](dart-lang/setup-dart@6a218f2...a57a6c0)

---
updated-dependencies:
- dependency-name: dart-lang/setup-dart
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

pkgs/appengine/.github/workflows/build.yaml

@@ -15,7 +15,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@755da8c3cf115ac066823e79a1e1788f8940201b
-      - uses: dart-lang/setup-dart@6a218f2413a3e78e9087f638a238f6b40893203d
+      - uses: dart-lang/setup-dart@a57a6c04cf7d4840e88432aad6281d1e125f0d46
 
       - name: Install dependencies
         run: dart pub get