fix nix-build trouble after fresh install:

problem:
Nix only recognizes nixbld users which have nixbld set in extraGroups.
After installation the user was created by activate with group='nixbld'
and extraGroups=[]
After rebooting a conditional shell script fixed this by adding
extraGroups=["nixbld"].

This patches creates the user they way it was in the past with
group='nobody' and extraGroups=["nixbld"].

Another solution would be making nix be aware of the primary group as
well. However this would require everyone to update Nix after updating
NixOS.

svn path=/nixos/trunk/; revision=18985

modules/config/users-groups.nix

@@ -32,8 +32,8 @@ let
              calls in `libstore/build.cc', don't add any supplementary group
              here.  */
           uid = builtins.add ids.uids.nixbld nr;
-          group = "nixbld";
+          group = "nogroup";
-          extraGroups = [];
+          extraGroups = ["nixbld"];
         };
 
       nixBuildUsers = map makeNixBuildUser (pkgs.lib.range 1 10);
@@ -213,16 +213,6 @@ in
                     --groups "$extraGroups" \
                     ''${home:+--home "$home"} \
                     --shell "$shell"
-                if test -z "$extraGroups"
-                then
-                    # Make sure the user is listed as belonging to its
-                    # primary group when it has no supplementary groups.  The
-                    # main reason is to have the `nixbld[0-9]' users be
-                    # listed as `nixbld' members; this allows `nix-store' to
-                    # get the UIDs of all the build users by doing a
-                    # getprnam("nixbld") call.
-                    groupmod "$group" -A "$name"
-                fi
             fi
 
         done