Do not search qemu binary when docker is in use
When executing a sparc64 qemu job with docker, LAVA throws an infrastructure error by not finding the qemu binary. This is normal since the binary will be in the docker image.
So when actions/boot/qemu is used with the optional docker, dont try to find the qemu binary.
Signed-off-by: Corentin LABBE clabbe@baylibre.com