Use correct path for flutter in flutter_gallery_v2_chrome_run_test.dart

[pennzht]

Description

The test flutter_gallery_v2_chrome_run_test is failing, because flutter is not on the PATH on devicelab hosts.
This PR tells the testing machine the location of flutter using a relative path.

Related Issues

Closes #55479

Tests

No tests have been added for this change.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
  • This is a fix; no new documentation was needed.
• All existing and new tests are passing.
  • flutter_gallery_v2_chrome_run_test.dart is not imported into any other files under devicelab, so this test should not affect other tests.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read Handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.

[rami-a]

LGTM

[jmagman]

LGTM, thanks!

[jonahwilliams]

LGTM