Fix non-existent variables

scripts/Common/OS/setup.fish

@@ -79,10 +79,8 @@ function runSetup
         end | runChroot "$mountDir" tee /root/.bash_profile >/dev/null
     end
 
-    function prepareNix -V dir
+    function prepareNix -V dir -V mountDir
         source "$dir/../../lib/nix.fish"
-        source "$dir/../../lib/settings.fish"
-        set -l mountDir (getOSConfig partition.rootDir)
 
         # Copy `nixpkgs` channel
         mkdir -p (dirname "$mountDir/$nixPkgsDir")