CI: Use workflow run's event commit SHA for JS benchmarks

Chaining workflows does not cause the subsequently spawned workflow runs
to use the same event, but rather it uses the latest head SHA based on
the branch it runs on. This would cause the JS benchmarks jobs to not be
able to find artifacts (if a new JS repl workflow was started before the
previous one could finish) and/or assign the wrong commit SHA to the
benchmark results.

Since `github.event` contains information about the original workflow
run that spawned the JS benchmarks jobs, we can take the commit SHA from
there and use it to download the correct artifact.

.github/workflows/js-benchmarks.yml

@@ -46,7 +46,7 @@ jobs:
         uses: dawidd6/action-download-artifact@v9
         with:
           workflow: js-artifacts.yml
-          commit: ${{ github.sha }}
+          commit: ${{ github.event.head_sha }}
           name: ladybird-js-${{ matrix.package_type }}
           path: js-repl
 
@@ -76,7 +76,7 @@ jobs:
         shell: bash
         run: |
           echo '{
-              "commit": "${{ github.sha }}",
+              "commit": "${{ github.event.head_sha }}",
               "os": "${{ matrix.os_name }}",
               "arch": "${{ matrix.arch }}",
               "artifact": "js-benchmarks-results-${{ matrix.os_name }}-${{ matrix.arch }}"