Add host-spawn and host tty access permissions

I use Terminus in conjunction with Sublime Text, and currently to get Terminus to work with the host system's shell (so that it can access my gpg keys, etc.)

Adding `flatpak-spawn --host` does not properly pass tty, etc. to the terminal instance. [host-spawn](https://github.com/1player/host-spawn) solves these issues. So adding `host-spawn` to the installation so that it's easier to get a host shell running would be nice.