[RISCV][lit-on-qemu] Rework implementation to just use ssh

The current implementation has a slightly awkward communication protocol
including files in a shared directory. Using the approach I document
here
<https://muxup.com/2026q2/bootable-qemu-image-menagerie-with-rootless-debootstrap>
in terms of ssh key injection and Unix domain sockets, we instead just
use ssh. Overall this should improve robustness and by virtue of being
more conventional, maintainability.