Set env to vulkan1.1 for spirv-as in test_shaders.py
Set env to vulkan1.1 for spirv-as in test_shaders.py

Fixes #968

KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross