Checkout github.head_ref for PRs

actions/checkout#27 (comment)
actions/checkout#1108

Author: br3ndonland

action.yml

@@ -104,7 +104,7 @@ runs:
       # Set repo and ref from which to run Docker container action
       # to handle cases in which `github.action_` context is not set
       # https://github.com/actions/runner/issues/2473
-      REF=${{ env.ACTION_REF || github.ref_name }}
+      REF=${{ env.ACTION_REF || github.head_ref || github.ref_name }}
       REPO=${{ env.ACTION_REPO || github.repository }}
       echo "ref=$REF" >>"$GITHUB_OUTPUT"
       echo "repo=$REPO" >>"$GITHUB_OUTPUT"