nixos/nsswitch: use symlinkJoin instead of makeLibaryPath to build sy…

…stem.nssModules.path

This makes this point to a single folder, not a colon-separated list of
directories, which makes it much easier to symlink to it (what the
next commit does).

This makes overriding *already existing* NSS modules harder, as we can't
just pretend to the list, but it's probably a good idea to explicitly
handle this, instead of silently shadowing - plus, I'm not aware of
anything in nixpkgs actually overwriting existing NSS modules.

nixos/modules/config/nsswitch.nix

@@ -14,14 +14,18 @@ with lib;
       internal = true;
       default = [];
       description = ''
-        Search path for NSS (Name Service Switch) modules.  This allows
-        several DNS resolution methods to be specified via
+        Path containing NSS (Name Service Switch) modules.
+        This allows several DNS resolution methods to be specified via
         <filename>/etc/nsswitch.conf</filename>.
       '';
       apply = list:
         {
           inherit list;
-          path = makeLibraryPath list;
+          path = pkgs.symlinkJoin
+            {
+              name = "nss-modules";
+              paths = list;
+            } + "/lib";
         };
     };