nscd service: fix ordering and start automatically

During working on #150837 I discovered that `google-oslogin` test
started failing, and so did some of my development machines. Turns out
it was because nscd doesn't start by default; rather it's wanted by
NSS lookup targets, which are not always fired up.

To quote from section on systemd.special(7) on `nss-user-lookup.target`:

> All services which provide parts of the user/group database should be
> ordered before this target, and pull it in.

Following this advice and comparing our unit to official `sssd.service`
unit (which is a similar service), we now pull NSS lookup targets from
the service, while starting it with `multi-user.target`.

nixos/modules/services/system/nscd.nix

@@ -50,7 +50,9 @@ in
     systemd.services.nscd =
       { description = "Name Service Cache Daemon";
 
-        wantedBy = [ "nss-lookup.target" "nss-user-lookup.target" ];
+        before = [ "nss-lookup.target" "nss-user-lookup.target" ];
+        wants = [ "nss-lookup.target" "nss-user-lookup.target" ];
+        wantedBy = [ "multi-user.target" ];
 
         environment = { LD_LIBRARY_PATH = nssModulesPath; };