Is your feature request related to a problem? Please describe.

Sometimes it can be cumbersome to refactor a derivation to 'pass in' a flake input, for example when you're working with a 'classic' nix system or the derivation you want to modify is somewhere 'deep'. In these cases the getFlake builtin comes in handy.

getFlake, however, strictly honors the flake.lock of whatever you got. If that refers to something big, such as nixpkgs, this might lead to different versions of that dependency in your store.

Describe the solution you'd like

It would be nice to be able to 'pass in' inputs to getFlake, similar to input.nixpkgs.follows = "nixpkgs" in flake.nix or --override-input on the nix commandline.

Priorities

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