Fix drone_dimension host_engine_builder. (flutter/engine#42068)

The host engine was using a top level dimension instead of a drone dimension and was causing the dart-internal build to fail.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/.ci.yaml

@@ -285,11 +285,11 @@ targets:
   - name: Linux linux_host_engine
     recipe: engine_v2/engine_v2
     timeout: 60
-    dimensions:
-      os: "Linux"
     properties:
       release_build: "true"
       config_name: linux_host_engine
+    drone_dimensions:
+      - os=Linux
 
   - name: Linux linux_host_desktop_engine
     recipe: engine_v2/engine_v2