Specific Env Vars and Edit Command

[Burtannia]

Description

Specific Env Vars

1. Introduce two new environment variables / arguments: NH_OS_FLAKE and NH_HOME_FLAKE to set the flake for the NixOS and Home Manager configs respectively.
2. Remove the FLAKE argument since it's no longer needed and, as others have pointed out #88, it's not very specific. Note that a fallback is in place to FLAKE if NH_..._FLAKE is not set so as to make the change a bit more backwards compatible.
3. Fallback to current directory if neither NH_..._FLAKE nor FLAKE is set.

Edit Command

Running nh <os | home> edit uses the aforementioned env vars to open the default editor (as specified by EDITOR) in the flake directory.