feat(common): add functionality to query the nix config

[jalil-salame]

This allows us to tackle issue #4 (nix cache detection).

This is intended to be used as follows:

use common::nix_conf;

// Helper that invokes `nix config show` and retrieves the stdout
let conf: String = nix_conf::nix_conf_get_from_cli().unwrap();

// Query the config for the "substituters" key (auto-merges extra-substituters, but it is not required)
let value: String = nix_conf::get_value(&conf, "substituters").unwrap();

// substituters is a list of URLs, parse the value like that
let substituters: Vec<&str> = value.split(' ').collect();

Some shortcuts were taken as we assume we are parsing nix config show's output. Specifically:

• include and !include are not resolved.
• We don't query the nix.conf search path for where to find the configuration files
• We don't have functionality to parse the whole configuration (as only substituters is needed for issue #4)

If the above mentioned complexity is needed I can implement it here or in a separate PR.