[ci] Only label on PR open (facebook#32936)

No reason to label it every update, only do it once when it's first
opened.
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[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/facebook/react/pull/32936).
* __->__ facebook#32936
* facebook#32935

Author: poteto

.github/workflows/shared_label_core_team_prs.yml

@@ -2,6 +2,7 @@ name: (Shared) Label Core Team PRs
 
 on:
   pull_request_target:
+    types: [opened]
 
 permissions: {}