fix: use triggering workflow branch for infra workflow

[shaspitz]

mev-commit-infra.yml workflow currently runs with main even if triggered from a PR branch. This workflow should instead use the branch of the triggering workflow.

Relevant docs:

• https://docs.github.com/en/webhooks/webhook-events-and-payloads#workflow_run
• https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_run