[CP-stable]Skip running Linux fuchsia_test on non-master channel. (#169380)

This pull request is created by [automatic cherry pick workflow](https://github.com/flutter/flutter/blob/main/docs/releases/Flutter-Cherrypick-Process.md#automatically-creates-a-cherry-pick-request)
Please fill in the form below, and a flutter domain expert will evaluate this cherry pick request.

### Issue Link:
What is the link to the issue this cherry-pick is addressing?

https://github.com/flutter/flutter/issues/169101

### Changelog Description:
Explain this cherry pick in one line that is accessible to most Flutter developers. See [best practices](https://github.com/flutter/flutter/blob/main/docs/releases/Hotfix-Documentation-Best-Practices.md) for examples

Reduces the cost of running the (sometimes flaky) `Linux fuchsia_test` on release branches.

### Impact Description:
What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)? Does it impact development (ex. flutter doctor crashes when Android Studio is installed), or the shipping production app (the app crashes on launch)

N/A - CI infra optimization.

### Workaround:
Is there a workaround for this issue?

N/A - CI infra optimization

### Risk:
What is the risk level of this cherry-pick?

### Test Coverage:
Are you confident that your fix is well-tested by automated tests?

### Validation Steps:
What are the steps to validate that this fix works?

N/A - CI infra optimization

engine/src/flutter/.ci.yaml

@@ -163,6 +163,9 @@ targets:
   - name: Linux linux_fuchsia_tests
     recipe: engine_v2/engine_v2
     timeout: 90
+    enabled_branches:
+      # Don't run this on release branches
+      - master
     properties:
       config_name: linux_fuchsia_tests
     # Do not remove(https://github.com/flutter/flutter/issues/144644)