feature: respect flake's `nixConfig` for `--check-cache-status` & `--flake`

[nrdxp]

A lot of flakes set local substituters in the flake.nix directly, and if we are going to use the json to decide whether or not to build a derivation we should use this project specific caches directly, at least when evaluating a flake with --flake.

It might also be useful to list which caches the derivation exists in to help in cases like #139