Use binDir in PATH

Makes the config a little more portable, but makes it harder to work on
binscripts (you'd have to 'hey re' on every change). I have another way
around that, but I'll push that later.

modules/options.nix

@@ -70,7 +70,7 @@ with lib.my;
 
     # must already begin with pre-existing PATH. Also, can't use binDir here,
     # because it contains a nix store path.
-    env.PATH = [ "$XDG_CONFIG_HOME/dotfiles/bin" "$PATH" ];
+    env.PATH = [ binDir "$PATH" ];
 
     environment.extraInit =
       concatStringsSep "\n"