action: fix checkout on pull_request_target

The `pull_request_target` trigger will checkout the master branch
codes by default, but we need to use the new PR codes on smoke test.

See: https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target

Signed-off-by: Yan Song <imeoer@linux.alibaba.com>

.github/workflows/smoke.yml

@@ -4,7 +4,7 @@ on:
   push:
     branches: ["**", "stable/**"]
     paths-ignore: [ '**.md', '**.png', '**.jpg', '**.svg', '**/docs/**' ]
-  pull_request_target:
+  pull_request:
     branches: ["**", "stable/**"]
     paths-ignore: [ '**.md', '**.png', '**.jpg', '**.svg', '**/docs/**' ]
   schedule: