Add CI

[niccoloraspa]

This CI adds a simple workflow to build mkosi images.

By default, it runs on every push to main, but it can also be triggered on a custom branch.
It is suited for bob-l1 and bob-l2 images (even though bob-l2 it's not on main right now :P).

There is definitely room for improvement, but it’s a good starting point.

Possible improvements:

• Improve build time (bob-l1 and bob-l2 are built in parallel right now but it could make sense to build them sequentially)
• Run image build only when changes are present
• Caching
• Upload artifacts to S3
• Calculate measurements
• Reproducibility tests as in trunk/buildernet
• Smoke tests on image

Test run on my fork: https://github.com/endlesschains/flashbots-images/actions/runs/21524492215