We need a way to set environment variables on a per-miner basis
GPU_FORCE_64BIT_PTR is causing problems.  This mostly seems to affect AMD cards.

Some miners *need* it set to 1.  Other *need* it set to 0.

I think we need to add another property to the miners for environment variables that need to be set, then when the job is created in the StartMining() function set those variables inside the job where they should only affect that one process.