ci: revert to pull_request (#757)

Author: xrkffgg

.github/workflows/preview.yml

@@ -1,7 +1,7 @@
 name: 🔂 Surge PR Preview
 
 on:
-  pull_request_target:
+  pull_request:
     types: [labeled]
 
 jobs:
@@ -10,8 +10,6 @@ jobs:
     if: contains(github.event.pull_request.labels.*.name, 'SurgePreview')
     steps:
       - uses: actions/checkout@v2
-        with:
-          ref: refs/pull/${{ github.event.pull_request.number }}/merge
       - uses: afc163/surge-preview@v1
         with:
           surge_token: ${{ secrets.SURGE_TOKEN }}