refactor(nix): improve flake (#1866)

I saw pkgs.stdenv.hostPlatform.system
I delete pkgs.stdenv.hostPlatform.system

- dropped forAllSystem in favor of pkgsOf.${system} + eachSystem.
  This lets us just have a ${system} in place of pkgs.stdenv.hostPlatform.system
- improved homeManagerModules import using [importApply](https://noogle.dev/f/lib/modules/importApply/)
- pruned redundant systems in platforms.linux by intersecting it with
  systems.flakeExposed

That's it for this commit message.
ROCK & STONE!!