Fix Clippy ref yet again

Author: jonas-schievink

.github/workflows/clippy.yml

@@ -9,8 +9,13 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+        if: github.event_name == 'pull_request_target'
         with:
-          ref: ${{ github.head_ref || github.ref }}
+          ref: refs/pull/${{ github.event.number }}/head
+      - uses: actions/checkout@v2
+        if: github.event_name != 'pull_request_target'
+        with:
+          ref: ${{ github.head_ref }}
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal