Build installer iso from unstable nixpkgs.

[Maj-majormajormajor]

Is your feature request related to a problem? Please describe.
To run nixos-install --flake directly from an installer iso, the --impure argument is required. The unstable version of "${modulesPath}/installer/cd-dvd/installation-cd-minimal.nix" provides this argument, it isn't in nixos. I want to build an installer iso that has the unstable version of nixos-install.

Describe the solution you'd like
I don't know how to do this within the design of this template. I'm pretty new to nix, so my limited understanding of this template may very well be wrong.

AFAIKT, the only mechanism to use packages from unstable is via the overlays mechanism. I don't know how that relates to directly importing the installer-cd-minimal.nix. I tried to find what package provides nixos-install, but didn't get very far in the nix source code.

I managed to work around this in a very hacky way. I replaced all references to nixos with master.

Maybe a module can be added that provides an installer.enable option along with an installer.fromUnstable option.

[nrdxp]

I've been meaning to add a simple mechanism for module override from master, similar to packages; it's almost done. Should have a PR soon.

[blaggacao]

For what it's worth, doing the same for sdImage requires to emulate renaming of util linux see specifically this change.

This change might also affect other installers.

[blaggacao]

Unfortunately an overlay of the form:

final: prev: {
  util-linux = prev.utillinux;
}

doesn't do the trick since this overlay is incompatible with master (since overlays are applied on both, nixos & master), see:

[flk]$ flk sd
error: --- EvalError ---------------------------------------------------------------------------------------------- nix
at: (657:15) in file: /nix/store/6l6hv9pyfmznqmxkk77bbig0a9wsqslp-source/pkgs/top-level/aliases.nix

   656|   usbguard-nox = usbguard; # added 2019-09-04
   657|   utillinux = util-linux; # added 2020-11-24
      |               ^
   658|   uzbl = throw "uzbl has been removed from nixpkgs, as it's unmaintained and uses insecure libraries";

infinite recursion encountered

[blaggacao]

for general's public information, I'm drawing a very distinct line on avoiding nixpkgs version conflicts on my attempt to extract the implicit abstract interfaces hidden in this repo out into flake-utils.