fix(ci): remove Gradle cache mount from pubGetLayer()

flutter pub get is pure Dart — it never invokes Gradle. The mutable
gradle-cache volume mount caused the same execution-cache instability
we just fixed for the pub cache: Dagger sees a changed volume and
cache-misses pubGetLayer() on every run.

The Gradle cache stays in Base(), which is only used for steps that
actually build Android code.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ci/main.go

@@ -216,7 +216,6 @@ func (m *Ci) pubGetLayer() *dagger.Container {
 		Include: []string{"pubspec.yaml", "pubspec.lock"},
 	})
 	return m.toolchain().
-		WithMountedCache("/home/ci/.gradle", dag.CacheVolume("gradle-cache"), dagger.ContainerWithMountedCacheOpts{Owner: "ci"}).
 		WithDirectory("/src", pubspecOnly, dagger.ContainerWithDirectoryOpts{Owner: "ci"}).
 		WithWorkdir("/src").
 		WithExec([]string{"/bin/bash", "-c",