Check out the right ref

Author: jonas-schievink

.github/workflows/clippy.yml

@@ -9,6 +9,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          ref: refs/pull/${{ github.event.number }}/head
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal