[CP-beta]Only build and download Fuchsia artifacts on master (#168169)

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/168088

### 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

N/A

### 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: Infrastructure only change.

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

N/A

### 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.yaml

@@ -836,6 +836,9 @@ targets:
   - name: Linux fuchsia_precache
     recipe: flutter/flutter_drone
     timeout: 60
+    enabled_branches:
+      # Don't run this on release branches
+      - master
     presubmit: false
     properties:
       shard: fuchsia_precache

engine/src/flutter/.ci.yaml

@@ -143,6 +143,9 @@ targets:
   - name: Linux linux_fuchsia
     recipe: engine_v2/engine_v2
     timeout: 120
+    enabled_branches:
+      # Don't run this on release branches
+      - master
     properties:
       release_build: "true"
       config_name: linux_fuchsia