Prevent double upload of benchmarks. (#42683)

The benchmarks have been migrated to v2 but the legacy build is uploading benchmarks even when running on staging.

Bug: flutter/flutter#127678

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

Author: godofredoc

.ci.yaml

@@ -190,7 +190,7 @@ targets:
     presubmit: false
     properties:
       build_host: "true"
-      upload_metrics: "true"
+      upload_metrics: "false"
     timeout: 60
 
   - name: Linux linux_benchmarks