fix(packaging): restrict nix-nswrapper to Linux platforms

[roberth]

It is only supported on that host platform.
This adds a bunch of redundant-ish wiring logic, because the alternative
is accessing meta.platforms generically, but that would cause these
flake output attrsets containing the packages to be strict in all
packages, which is very bad for performance and robustness.

Checked:

• Cross-compilation still works for Linux targets (arm, riscv),
  even when build platform is Darwin
• No attributes for nix-nswrapper for native Darwin

[xokdvium]

Not a fan of how this complicates things even further than they are. I wonder if there's a way to simplify this stuff. Probably not a lot of reason to keep this stuff in a separate component - might as well be an executable in the nix-cli component tbh.

[Ericson2314]

If it was an executable in the nix-cli component, that would give it a bunch of unnecessary nix-level dependencies, fwiw. We want to be use it is only linking libutil, because otherwise we'll run into those multithreading problems again.

[xokdvium]

We want to be use it is only linking libutil, because otherwise we'll run into those multithreading problems again.

Have a separate executable in that component?

[roberth]

Maybe the overhead of evaluating all components' meta.stuff isn't horrible. If so, you'd only degrade robustness a little.
Anyway, this is the low-risk way of fixing this and master is currently broken.