Checkout correct branch on pull_request_target trigger

Signed-off-by: Fynn Schmitt-Ulms <fschmitt@redhat.com>

Author: fynnsu

.github/workflows/smoke.yaml

@@ -4,11 +4,7 @@ name: "Run smoke tests via Tox::pytest"
 # These tests will be long running and require accelerated hardware.
 
 on:
-  workflow_dispatch:
-    inputs:
-      branch:
-        type: string
-        default: main
+  workflow_dispatch: {}
   # using this rather than pull_request because this workflow
   # needs to run in the context of the base branch (main) and
   # access the repo's secrets to start the AWS instances.
@@ -123,11 +119,16 @@ jobs:
           export PATH="${PATH}:${CUDA_HOME}/bin"
           nvidia-smi
 
-      - name: "Checkout code"
+      - name: Checkout
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
           fetch-depth: 0
-          ref: ${{inputs.branch}}
+
+      - name: Fetch and checkout PR
+        if: ${{ github.event_name == 'pull_request_target'}}
+        run: |
+          git fetch origin pull/${{ github.event.number }}/head:pr-${{ github.event.number }}
+          git checkout pr-${{ github.event.number }}
 
       # installs in $GITHUB_WORKSPACE/venv.
       # only has to install Tox because Tox will do the other virtual environment management.