[actions] switch Automatic Rease workflow to pull_request_target event

Author: ljharb

.github/workflows/rebase.yml

@@ -1,6 +1,6 @@
 name: Automatic Rebase
 
-on: [pull_request]
+on: [pull_request_target]
 
 jobs:
   _:
@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
     - uses: ljharb/rebase@master
       env:
         GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}