feat: Add nixosConfig option to nixosGenerate

[shymega]

WIP! WIP! WIP!

Currently doesn't work, but the idea is for an extra option (but not required) to nixosGenerate - a nixosSystem, which is then merged with the image attrset.

[Mic92]

I don't think this is needed. You can use the modules parameter to pass in your customizations. Also would needs to use imports so it merges correctly.

[shymega]

Sorry, meant to explain further in PR. I've tried passing through my nixosConfiguration as a module, and I get the following error:

nix build -L .\#packages.MTX-SRV
error:
       … while evaluating the attribute 'config.system.build."${(image).config.formatAttr}"'

         at /nix/store/5z81la6av4j0ckp0w1949lxiwjyyykks-source/lib/modules.nix:322:9:

          321|         options = checked options;
          322|         config = checked (removeAttrs config [ "_module" ]);
             |         ^
          323|         _module = checked (config._module);

       … while calling the 'seq' builtin

         at /nix/store/5z81la6av4j0ckp0w1949lxiwjyyykks-source/lib/modules.nix:322:18:

          321|         options = checked options;
          322|         config = checked (removeAttrs config [ "_module" ]);
             |                  ^
          323|         _module = checked (config._module);

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: Could not load a value as a module, because it is of type "configuration", in file /nix/store/5z81la6av4j0ckp0w1949lxiwjyyykks-source/flake.nix. If you do intend to import this configuration, please only import the modules that make up the configuration. You may have to create a `let` binding, file or attribute to give yourself access to the relevant modules.
       While loading a configuration into the module system is a very sensible idea, it can not be done cleanly in practice.

I thought my PR might be a good starting point to get this working.

The source is here

[Mic92]

If you just want to generate for your nixos configuration than import nixos-generators.nixosModules.all-formats in your nixos configuration and than reference
nixosConfigurations.my-machine.config.formats.proxmox instead. One cannot import a nixos systems like that as a module. There is an example for that in the README.

[shymega]

Right, okay. Should I close the PR? It would be nice to be able to pass a nixosConfiguration to nixosGenerate, but I understand (as a maintainer) if you'd rather keep the current functionality.

[shymega]

I think #246 implements this anyway. My preference would be to let the community vote on whenever they'd like the proposed functionality. As this PR is a dupe of #246, closing as dupe.