fix(pm/josh): fix josh ssh forwarding

I think this may be due to the recent 25.11 updates, but I'm not
entirely sure.

sshd started to create forwarded agents under $HOME. This broke Josh
since as home was /var/empty, so forwarding was impossible, resulting in
this error:

debug1: Requesting authentication agent forwarding.

... 8< -- snip -- 8< ...

debug1: Remote: Agent forwarding disabled: couldn't create listener socket

Creating a home for Josh fixes this

packetmix/systems/teal/josh.nix

···
     isSystemUser = true;
     group = "git";
     shell = "${josh}/bin/josh-ssh-shell";
+    createHome = true;
+    home = "/var/lib/josh/home";
   };
   users.groups.git = { };