nfs-utils: set /etc/krb5.keytab as default path for rpc-gssd

Currently the `rpc-gssd.service` has a `ConditionPathExists` clause that can
never be met, because it's looking for stateful data inside `/nix/store`.

`auth-rpcgss-module.service` also only starts if this file exists.

Fixes NixOS/nixpkgs#29509.