eliminate userFlakeNixOS

[blaggacao]

this issue targets the develop branch

Currently userFlakeNixOS is explicitly passed around as the "main" nixpkgs version.

However, with the new API, we afford the notion of channels and each host defines (or falls back) to a well known channel name.

We can ensure that the channel input is aquired by scanning inputs for ones named after the channel name.
in this case, we can use channelName attributes to recover the correct nixpkgs to use in those occasions.

Not implementing this would lead to an incorrect implementation since a host selected channel might end up with the wrong nixpkgs in those places.

rg userFlakeNixOS

[...]

# goes away with usage of `fup`
devos/mkPkgs.nix
3:{ userFlakeNixOS, userFlakeSelf, userFlakeInputs }:
25:    { pkgs = lib.os.pkgImport userFlakeNixOS overlays system; }

# would better use fup-evaluated `host.chanelName` to derive the nixpkgs from `inputs.<channelName>` (presuming they are ensured to be named the same)
devos/devosSystem.nix
3:{ userFlakeNixOS, userFlakeSelf, userFlakeInputs }:
18:            "${userFlakeNixOS}/${modpath}/installer/cd-dvd/installation-cd-minimal-new-kernel.nix"

# use defaults from `hostDefaults.<channelName>`
mkFlake/evalArgs.nix
3:{ userFlakeSelf, userFlakeNixOS }:
54:            default = userFlakeNixOS;
195:                input = userFlakeNixOS;

# same as devosSystem
devos/mkHosts.nix
3:{ userFlakeNixOS, userFlakeInputs, userFlakeSelf }:
41:        "nixpkgs=${userFlakeNixOS}"
50:        nixos.flake = userFlakeNixOS;
100:      inherit userFlakeNixOS userFlakeInputs userFlakeSelf;

[blaggacao]

Implememted in api-next