lambda-mod-zsh-theme: refactor install behavior to use `themes` dir
lambda-mod-zsh-theme: refactor install behavior to use `themes` dir

Now it's possible to set the path of this derivation as `ZSH_CUSTOM` in
the zsh configuration, so it's possible to use the theme directly from
the store path.

see https://github.com/robbyrussell/oh-my-zsh/wiki/Customization#overriding-and-adding-themes

NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs