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

pkgs/shells/lambda-mod-zsh-theme/default.nix

@@ -16,8 +16,8 @@ in stdenv.mkDerivation {
   buildPhases = [ "unpackPhase" "installPhase" ];
 
   installPhase = ''
-    mkdir -p $out/share
-    cp lambda-mod.zsh-theme $out/share
+    mkdir -p $out/share/themes
+    cp lambda-mod.zsh-theme $out/share/themes
   '';
 
   meta = with stdenv.lib; {