Allow to choose system when running nix flake check

[yajo]

Is your feature request related to a problem? Please describe.
Trying to set up tests for a flake, if I use flake-utils to provide multiarch, nix flake check will fail always with similar messages:

error: a 'aarch64-darwin' with features {} is required to build '/nix/store/whaiyx9z56sw98nppklhiz8zw2pn4wqi-mrchefMealShallowRepo.drv', but I am a 'x86_64-linux' with features {benchmark, big-parallel, kvm, nixos-test}

Describe the solution you'd like
Some flag like nix flake check --current-system.

Describe alternatives you've considered
Not providing multiarch.

[TLATER]

I think this is a duplicate of #4265, arguably nix flake check should evaluate multiarch if you're specifying that your outputs should run on other architectures so I'm not sure this should be done if that bug is fixed in a different way.

[yajo]

Indeed, as I have an IFD in my code. Closing as duplicate.