Update label.yml (#4151)

modifying github action to add label new contributor opens a pr _only_
(not closes). this is a result of Leona flagging that since the vercel
migration, the new contributor label was being removed shortly after the
gha adds it. [example
pr](#4121)

cc @runleonarun

this might have something to do with the 'authorize in vercel' workflow
perhaps?

.github/workflows/label.yml

@@ -2,7 +2,7 @@ name: Add/Remove Labels
 
 on:
   pull_request_target:
-    types: [ opened, closed ]
+    types: [ opened ]
 
 jobs:
   add_new_contributor_label: