Re-enable plugins_analyze. (#96977)

The test was disabled for release branches because there were some
failures that got attributed to checking plugins from master.

Checking the logic of the test more closely it is already using a pinned
version of the plugins repository and it should be consistent on
pre/post submit validations.

Bug: https://github.com/flutter/flutter/issues/91935

.ci.yaml

@@ -353,9 +353,6 @@ targets:
   - name: Linux flutter_plugins
     recipe: flutter/flutter_drone
     timeout: 60
-    enabled_branches:
-      - main
-      - master
     properties:
       shard: flutter_plugins
       subshard: analyze