Add python as system dependency properly

[Klaim]

Use the recent build2 features to allow this, enabling python install in the build2's CI

[Klaim]

This is partially implemented in main but not yet ready.
The heart of the issue is that different linux distributions will provide python 3 packages either as python or python3 and also have inconsistent python/3 executable naming.
This is partially handled here but is incomplete. Help is welcome.