Lagarto OX support

See merge request rarias/nixos-riscv!1

This may be used by another script to determine where we managed to
boot.

No need to run the SPEC benchmarks all the time. Stopping at the stage2
shell is enough to test most of the system.

We use the ENABLE_SPI toggle to also set the PLIC number of inputs.

Now we can see jobs from other users.

It seems to be hanging now in LVM.

The one contained in the CPU SPEC ISO is built for x86, and there is no
binary for RISC-V.