refactor: pushing Nix out of scope

[xynydev]

Why?

• The main purpose of startingpoint is to be a template repository, not providing extra features that do not enhance the custom image creation experience.
• I as the main maintainer of this repo have no knowledge about Nix, relying on other people's testing and word

Alternatives

There are two paths you can go by, but in the long run, there's still time to change the road you're on.

Pushing upstream

A Nix installer is an useful feature that could be included in the ublue-os/config repo and ingested by ublue-os/main and the children.
This would require a proposal and someone to just do it, after which this repo would be cleaned up.

Pushing downstream

A Nix installer is one of those thing not everybody needs and could be made a frontrunner in the quest to create a "patch-based" system for creating custom images (see #94).
This would require somebody to step up as the maintainer of the nix-installer patch and sketch out with me the plan for how such a patch system would look. I'd also certainly help getting the repo started if needed, but keep my hands of the thing.

[xynydev]

Discussed at length in #85, decided to push upstream. Discussing in closed PRs gets lost quickly, but I appreciate the discussion that went on. Unless somebody else gets first I'll be doing this maybe tomorrow.

[xynydev]

blue-build/modules#2 is now complrted so we should be able to use that. Setting this as a TODO for myself.