ci: bench on PR sha

Author: MilesCranmer

.github/workflows/benchmark_pr.yml

@@ -10,4 +10,7 @@ jobs:
       - uses: MilesCranmer/AirspeedVelocity.jl@action-v1
         with:
           julia-version: "1"
-          job-summary: "true"  # Post to "summary" tab of workflow run
+          # Post to "summary" tab of workflow run:
+          job-summary: "true"
+          # Run benchmark using PR's version of the script:
+          bench-on: ${{ github.event.pull_request.head.sha }}