fix: replace deprecated nixpkgs.hostPlatform with nixpkgs.system

[Dichgrem]

nixpkgs.hostPlatform is deprecated and triggers evaluation warnings:

evaluation warning: 'hostPlatform' has been renamed to/replaced by 'stdenv.hostPlatform'

Replace with nixpkgs.system in lib/default.nix lines 193, 199, 205.

Verified that nixpkgs.system evaluates correctly in a NixOS module context.

[Dichgrem]

Note: The CI failure (fileSystems."/".fsType) is a pre-existing issue unrelated to this change, also present in previous PRs (e.g. #148).

[r-vdp]

Thanks for the PR, but I think this is chasing the wrong attribute.

The warning you quoted

'hostPlatform' has been renamed to/replaced by 'stdenv.hostPlatform'

is the alias warning for pkgs.hostPlatform — the top-level package-set attribute (pkgs/top-level/aliases.nix). Blueprint reads the NixOS option config.nixpkgs.hostPlatform, which is a different thing and is not deprecated — it's the option NixOS recommends setting (the nixpkgs.system option's own error message says "will be deprecated in the future, so we recommend to set nixpkgs.hostPlatform"). Whatever is emitting that warning in your config is something reading pkgs.hostPlatform, not blueprint.

Switching to config.nixpkgs.system actually regresses things: that option's default throws when only hostPlatform is set (which is what blueprint's templates and docs tell users to do):

$ nix eval .#nixosConfigurations.my-nixos.config.nixpkgs.system
error: Neither nixpkgs.system nor any other option in nixpkgs.* is meant
to be read by modules and configurations.
Use pkgs.stdenv.hostPlatform instead.

The template's toplevel still built in your testing only because nothing in it consumes perSystem; any host module that does perSystem.<anything> would hit that throw.

If we wanted to follow nixpkgs' guidance to the letter, the spelling would be pkgs.stdenv.hostPlatform.system rather than config.nixpkgs.system — but that's a separate issue, not a fix for the warning you saw, so I'll close this one.

(The unrelated fsType CI failure you noted is fixed on main by #151.)