nixos/sks: Use a group and don't add sks to systemPackages

Without a group the gid will default to 65534 (2^16 - 2) which maps to
"nogroup". IMO it makes more sense to explicitly set a valid group.

Adding pkgs.sks to environment.systemPackages is not required (IIRC we
want to avoid bloating environment.systemPackages). Instead it seems
like a better idea to make the relevant binaries available to the user
sks and enable useDefaultShell so that "su -l sks" can be used for
manual interaction (that way the files will always have the correct
owner).