Add a workflow that attaches to Bencher bare metal jobs

When a detached bare metal run finishes, its callback sends a
`bencher_run` repository_dispatch. This workflow attaches to that Job
with `bencher run --job`, then completes the GitHub Check the detached
run started and posts the pull request comment.

GitHub only runs repository_dispatch workflows from the default branch,
so this file has to live on main. Until job callbacks ship, it installs
the CLI from the job callbacks branch, cached by commit, and talks to
the dev API.

Author: epompeii

.github/workflows/bare_metal_attach.yml

@@ -0,0 +1,50 @@
+on:
+  repository_dispatch:
+    types: [bencher_run]
+
+jobs:
+  attach_pr_benchmarks:
+    name: Post PR Benchmark Results from Bencher Bare Metal
+    permissions:
+      checks: write
+      pull-requests: write
+    runs-on: ubuntu-latest
+    steps:
+      - name: Resolve the Bencher CLI commit
+        id: bencher-cli
+        run: |
+          sha="$(git ls-remote https://github.com/bencherdev/bencher refs/heads/u/ep/callback/smoke | cut -f1)"
+          test -n "$sha"
+          echo "sha=$sha" >> "$GITHUB_OUTPUT"
+          echo "key=bencher-cli-$ImageOS-$RUNNER_ARCH-$sha" >> "$GITHUB_OUTPUT"
+      - name: Restore the Bencher CLI
+        id: restore-bencher-cli
+        uses: actions/cache/restore@v5
+        with:
+          path: ~/.bencher-cli
+          key: ${{ steps.bencher-cli.outputs.key }}
+      - name: Install the Bencher CLI
+        if: steps.restore-bencher-cli.outputs.cache-hit != 'true'
+        env:
+          BENCHER_SHA: ${{ steps.bencher-cli.outputs.sha }}
+        run: cargo install --git https://github.com/bencherdev/bencher --rev "$BENCHER_SHA" --locked --root ~/.bencher-cli bencher_cli
+      - name: Save the Bencher CLI
+        if: steps.restore-bencher-cli.outputs.cache-hit != 'true'
+        uses: actions/cache/save@v5
+        with:
+          path: ~/.bencher-cli
+          key: ${{ steps.bencher-cli.outputs.key }}
+      - name: Add the Bencher CLI to the path
+        run: echo "$HOME/.bencher-cli/bin" >> "$GITHUB_PATH"
+      - name: Post PR Benchmark Results with Bencher
+        env:
+          JOB_UUID: ${{ github.event.client_payload.bencher.job }}
+          PROJECT_SLUG: ${{ github.event.client_payload.bencher.project }}
+        run: |
+          bencher run \
+          --host https://dev.api.bencher.dev \
+          --project "$PROJECT_SLUG" \
+          --token '${{ secrets.DEV_BENCHER_API_TOKEN }}' \
+          --job "$JOB_UUID" \
+          --error-on-alert \
+          --github-actions '${{ secrets.GITHUB_TOKEN }}'