flake check: shadow pkg on unsupported platform

[anntnzrb]

Describe the bug
I am using aarch64-darwin and I am unable to pass the nix flake check (which needs the impurity flag). It states the requirement of the shadow package, which is indicated to be os-specific (linux). I am thinking the template is in need of a tweak for this specific system.

To reproduce

1. Init using the currently offered flake-parts template:
   $ nix flake new test-devenv -t github:cachix/devenv#flake-parts

2. cd & run a flake check:
   $ nix flake check --impure .

3. Analyze logs

Version
Using flakes. Via the flake-parts template.