Change size labeling action's trigger event

Change to `pull_request_target`, so fork builds don't fail

.github/workflows/size-label.yml

@@ -1,14 +1,17 @@
 name: Size Label
 
 on:
-  pull_request:
-
-permissions:
-  contents: read
-  pull-requests: write
+  pull_request_target:
+    types:
+      - opened
+      - edited
+      - synchronize
 
 jobs:
   size-label:
+    permissions:
+      contents: read
+      pull-requests: write
     runs-on: ubuntu-latest
     steps:
       - name: size-label