fix: use the "state" directory in systemd unit files

As pointed out by @MrCowKing in, "${HOME}" isn't expanded. We use the
state directory as that's guaranteed to exist (and, in the default
configuration, will match the "HOME" directory of the ipfs user).

See: https://github.com/ipfs/kubo/commit/b47ecd0cba85905f18b85de9dd771008adf60fd0#commitcomment-83565661