feat: automate building of shorebird's engine in the cloud

[eseidel]

I could have sworn we had a bug on this already, but can't find it.

Basically finish the work started in:
https://github.com/shorebirdtech/build_engine
Except run those builds in the cloud, for Windows, Mac and Linux.

This is a prerequisite to solving #472

[eseidel]

We have Linux limping along. https://github.com/shorebirdtech/build_engine/actions/runs/5663271016
Will add Mac and Windows soon.