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

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

.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: {}