Support arbitrary flake attributes instead of `nixosConfigurations`

[srhb]

Prerequisites

• I have searched existing issues to make sure this isn't a duplicate
• I have checked the documentation to see if this feature already exists

Problem Description

Sometimes I don't want to use .#nixosConfigurations, because, for instance, I can't use nix flake check in a reasonable (life-)time if I have hundreds of machines in it.

I think it's more realistic to want nixos-anywhere to be less opinionated, rather than waiting for nix to get configurability for checks, or parallel eval, or (...)

Proposed Solution

Either don't assume nixosConfigurations or give me yet another flag to remove that assumption

Alternatives Considered

Use flake-compat and direct storepaths as a workaround.

Feature Type

New command-line option

Use Case

No response

Implementation Ideas

No response

Additional Context

No response