While doing kernel development with the nixos testing infrastructure, it
is useful to quickly compile the kernel on the side and boot it in the testing VM.
This patch allows overriding the kernel through the environment
variable. For example, the following:
$ NIXPKGS_QEMU_KERNEL_testvm=$LINUX_SRC/arch/x86/boot/bzImage $VM/bin/nixos-test-driver
runs testvm testing VM with the fresh kernel compiled in $LINUX_SRC.
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
