Add an expression, based on the nixos qemu virtualisation framework, for
a basic system whose nix is tvix, to be used for testing tvix
Change-Id: I3c7422bb10d3ce05a3094671cb770f1f745d814c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1423
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: Alyssa Ross <hi@alyssa.is>
Tested-by: BuildkiteCI