OpenCL_jll.jl should check privilege level

[maleadt]

On high integrity levels (i.e., running as Administrator), the env vars we set aren't picked up by the Khronos ICD loader: https://github.com/KhronosGroup/OpenCL-ICD-Loader/blob/92280246c8e30795f08362425510d6acfc5c3f0d/loader/windows/icd_windows_envvars.c#L43-L74

KHR ICD trace at /workspace/srcdir/OpenCL-ICD-Loader/loader/windows/icd_windows_envvars.c:69: Running at a high integrity level, so secure_getenv is returning NULL

This breaks OpenCL_jll.jl, so we should probably warn if we detect this situation.