chore(ci): update to install-nix-action@v20

switch console kernel param order...

...according to mic92 ordering does matter here.

remove kernel params for testing

switch to predictable interface names

set serial console...

...settings cargo culted from https://github.com/nix-community/nixos-images/blob/main/nix/kexec-installer/module.nix

load all modules by default...

...bit out of ideas why kexec doesn't work in the test runner tbh

add availableKernel modules...

include the same set as
https://github.com/nix-community/nixos-images/tree/main/nix/kexec-installer

remount root: add /tmp

add /etc/os-release...

because system.nixos.variant_id isn't in nixos-unstable yet

release kexecTarball for tags