Update PR benchmark to use -O3

Author: MilesCranmer

.github/workflows/benchmark_pr.yml

@@ -38,7 +38,7 @@ jobs:
                 echo $PATH
                 ls -l ~/.julia/bin
                 mkdir results
-                benchpkg ${{ steps.extract-package-name.outputs.package_name }} --rev="${{github.event.repository.default_branch}},${{github.event.pull_request.head.sha}}" --url=${{ github.event.repository.clone_url }} --bench-on="${{github.event.pull_request.head.sha}}" --output-dir=results/ --tune
+                benchpkg ${{ steps.extract-package-name.outputs.package_name }} --rev="${{github.event.repository.default_branch}},${{github.event.pull_request.head.sha}}" --url=${{ github.event.repository.clone_url }} --bench-on="${{github.event.pull_request.head.sha}}" --output-dir=results/ --tune --exeflags="-O3 --threads=auto"
             - name: Create plots from benchmarks
               run: |
                 mkdir -p plots