ci: add flashbots integration tests to CI

[init4samwise]

Summary

Adds make test-flashbots command to the Builder CI pipeline.

Changes

• Makefile: Added test-flashbots target that runs cargo nextest run --run-ignored=only --workspace to execute all ignored integration tests
• flashbots-ci.yml: New GitHub Actions workflow that runs the flashbots integration tests on pushes to main and PRs

Testing

The workflow installs cargo-nextest and Foundry, then runs the integration tests that are currently marked with #[ignore] annotations.

Closes ENG-1300

[dylanlott]

This should be a triggerable job in Github Actions, but should not be added to the main CI pipeline. The Flashbots integrations tests are too flaky to be in the hot path. Also not sure if they still pass post-Fusaka.

[init4samwise]

Thanks for the review @dylanlott! Good call - I've updated the workflow to use workflow_dispatch instead of running on push/PR, so it's now manually triggerable only.

Also added an optional ref input so you can test specific branches/commits when needed.

Re: post-Fusaka compatibility - that's a good point. We can run the workflow manually once this is merged to verify current state.

[init4samwise]

Hey @dylanlott — just following up on this. The workflow is now workflow_dispatch only (commit b92ee03), so it won't run in the main CI pipeline at all. You can trigger it manually from the Actions tab whenever needed.

Given your note about post-Fusaka flakiness, I kept it simple — no schedule or automatic triggers. Manual-only gives full control over when to run these tests.

Would you mind taking another look when you get a chance? 🙏