add flakeDir option to automatically set the FLAKE variable

[NotAShelf]

Adds a flakeDir option that sets the FLAKE variable for the user.

[viperML]

1. Why not call it flake instead of flakeDir? If you are concerned about the user setting a flake.nix, it can be asserted.

2. Type should be types.path

3. What if the user doesn't set it? The type should be nullOr path then.

[NotAShelf]

I was thinking an assertion instead of a nullOr - but that's not very backwards compatible. I'll make it nullOr.

[NotAShelf]

Okay, it now asserts if nh.flake points at a file that has the .nix prefix.

[viperML]

Checks fail

[NotAShelf]

I really don't know if that mkIf is the correct way to conditionally assert.

[viperML]

Why not?

assertion = (config.nh.flake != null) -> !(lib.hasSuffix ".nix" config.nh.flake)

[NotAShelf]

Mainly because I didn't know how the -> operator worked. Will change.

[viperML]

Is it ready?

[NotAShelf]

Will commit tonight.

[NotAShelf]

Should be good now.