nixos/public-inbox: set ProtectHome=tmpfs

This fixes using coderepos in /home, by allowing the coderepo paths to
be bind mounted into an otherwise empty /home tmpfs. Since this was
the usecase for making ProtectHome= overrideable, we don't need the
mkDefault any more.

nixos/modules/services/mail/public-inbox.nix

···
       PrivateNetwork = mkDefault (!needNetwork);
       ProcSubset = "pid";
       ProtectClock = true;
-      ProtectHome = mkDefault true;
+      ProtectHome = "tmpfs";
       ProtectHostname = true;
       ProtectKernelLogs = true;
       ProtectProc = "invisible";