Some upcoming changes might require the Nix build to be split into
multiple separate nix-build invocations of different expressions, thus
splitting this out is useful.
It also fixes an issue where `build-image/default.nix` might be called
in an environment where no Nix channels are configured.
ALl the ones except for build-image.nix are considered trivial. On the
latter, nixfmt makes some useful changes but by-and-large it is not
ready for that code yet.
Removes usage of the old layering algorithm and replaces it with the
new one.
Apart from the new layer layout this means that each layer is now
built in a separate derivation, which hopefully leads to better
cacheability.
Instead of requiring the server component to be made aware of the
location of the Nix builder via environment variables, this commit
introduces a wrapper script for the builder that can simply exist on
the builders $PATH.
This is one step towards a slightly nicer out-of-the-box experience
when using `nix-build -A nixery-bin`.
