Only build and download Fuchsia artifacts on master (#168154)

Closes https://github.com/flutter/flutter/issues/168089.

/cc @zanderso

.ci.yaml

@@ -835,6 +835,9 @@ 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