fix automated pr labeling (bevyengine#2316)

# Objective

- the PR labeler workflow will always add the `needs-traige` label to every pull request event

## Solution

- Limit this workflow to only be on the `opened` event

.github/workflows/label.yml

@@ -3,6 +3,8 @@ on:
   pull_request_target:
     branches:
       - main
+    types:
+      - opened
 
 jobs:
   label: