Vulkan: avoid loading Vulkan library multiple times #3424

Job	Run time
Linux -> Pre-checks	13s
Linux -> GCC-x64, Debug	11m 48s
Linux -> GCC-x64, Release	10m 39s
Linux -> Clang-x64, Debug	12m 59s
Linux -> Clang-x64, Release	12m 56s
Linux -> Clang-NO_GLSLANG-x64, Debug	7m 38s
Linux -> Clang-NO_GLSLANG-NO_HLSL-x64, Release	6m 23s
	1h 2m 36s

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