Exercise more USD build options

Either the script `scripts/vfx/build_usd.sh` or an eventual Conan USD package should exercise more compile time options as per the output of the configuration script:

```
-- Skipping validation of gf generated code because PXR_VALIDATE_GENERATED_CODE=OFF
-- Skipping alembic-based usddiff tests because PXR_BUILD_ALEMBIC_PLUGIN=OFF
-- Skipping Draco-based usddiff tests because PXR_BUILD_DRACO_PLUGIN=OFF
-- Skipping hgiMetal because PXR_BUILD_GPU_SUPPORT or PXR_ENABLE_METAL_SUPPORT is OFF
-- Skipping hgiVulkan because PXR_BUILD_GPU_SUPPORT or PXR_ENABLE_VULKAN_SUPPORT is OFF
```

Start with scripts/vfx/build_usd.sh and compare its options with the configuration output listed in the issue. Done means the selected build path exercises the named validation, Alembic, Draco, Metal, and Vulkan options where supported, with the resulting configuration checks no longer skipped.

Written by the indexing model from the issue text.