loader: Add an environment variable to override driver name choice.

loader: Add an environment variable to override driver name choice.

My vc4 simulator has been implemented so far by having an entrypoint
claiming to be i965, which was a bit gross.  The simulator would be a lot
less special if we entered through the vc4 entrypoint like normal, so add
a loader environment variable to allow the i965 fd to probe as vc4.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>