allow prompting for accepting flake config from `nix build`

[ToyVo]

If your flake has updated the flake config and we use nh os build we get output from nix build saying

warning: ignoring untrusted flake configuration setting 'extra-substituters'.
Pass '--accept-flake-config' to trust it
warning: ignoring untrusted flake configuration setting 'extra-trusted-public-keys'.
Pass '--accept-flake-config' to trust it

if you were to instead use nix build within the directory first, you would get prompted to accept the options (example)

do you want to allow configuration setting 'extra-substituters' to be set to 'https://cache.nixos.org https://nix-community.cachix.org https://toyvo.cachix.org https://nixcache.diekvoss.net' (y/N)? y
do you want to permanently mark this value as trusted (y/N)? y
do you want to allow configuration setting 'extra-trusted-public-keys' to be set to 'cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs= toyvo.cachix.org-1:s++CG1te6YaS9mjICre0Ybbya2o/S9fZIyDNGiD4UXs= nixcache.diekvoss.net:31yGqcL45nEZ73gLt569UoAopxWOfC5bactGqU9C9mI=' (y/N)? y
do you want to permanently mark this value as trusted (y/N)? y

I'm guessing that nix detects if its being run interactively to prompt vs warn, if we can pass through that state that would be nice.

[ToyVo]

this seems to be the code path in nix, I don't see an interactivity check, so the logger->ask must just use the default
https://github.com/NixOS/nix/blob/c5b1be46b45ecbc6113efccd5c23b6b44fae91f9/src/libflake/config.cc#L54-L74

I wonder if shell in place of cmd https://docs.rs/subprocess/latest/subprocess/struct.Exec.html#method.shell
in

nh/src/commands.rs

Line 228 in 003dd72

let base_command = Exec::cmd("nix")

would fix this

[NotAShelf]

This seems like an interesting addition, but I currently lack the bandwidth to look into it. I also have a patch for Nix that rejects flake configs outright. However, I'd appreciate a PR on the matter if you believe the addition is necessary.