Update nix.md

[Ryoschin]

• Separate the additional packages and additional libraries declaration in the home.nix section by using a with expression (which also removes the need to write inputs.ags.packages.${pkgs.system} every new library the user would want to add)

• Add an example for a library containing two uppercase letters in it's name (WirePlumber becomes wireplumber, not wirePlumber, or such)

[Aylur]

We can replace battery for wireplumber to make an example, but I don't like separating the two as it just adds unnecessary noise. We are showing how to work with ags and not teaching nix. Users can separate them themselves if they like.