Download NVAPI when DILIGENT_NVAPI_PATH is unset (close #773) #4274

Job	Run time
Linux -> Pre-checks	16s
Linux -> Clang-x64, Release	12m 57s
Linux -> GCC-x64, Release	11m 20s
Linux -> Clang-NO_GLSLANG-NO_HLSL-x64, Release	6m 49s
Linux -> Clang-NO_GLSLANG-x64, Debug	8m 5s
Linux -> GCC-x64, Debug	12m 19s
Linux -> Clang-x64, Debug	14m 45s
	1h 6m 31s

Provide feedback