Perform environment cleanup

[danielgafni]

I think this action has to be responsible for exiting the Nix environment as well. Otherwise it may interfere with Actions internals, such as running other cleanup steps.

There should be:

• a separate action to perform cleanup on-demand
  • or maybe a documented way to do this manually if it's robust and easy
• an automated cleanup step

An example job failure in system steps that clearly fails because of this (I had to include git into my dev shell because it has to be used with uv, I suspect this cause it to fail). But also please let me know if my flake.nix is incorrect.