Make sure to find vulkan if vulkan support is enabled in pxrConfig.cmake

[boberfly]

Description of Change(s)

Make sure to locate the Vulkan SDK if Vulkan support was enabled.

When building against OpenUSD, if hgiVulkan was built we need to locate where the vulkan SDK is as Vulkan::Vulkan is a target dependency in pxrTargets.cmake.

Link to proposal (if applicable)

Fixes Issue(s)

Checklist

• [x] I have created this PR based on the dev branch

• [x] I have followed the coding conventions

• [ ] I have added unit tests that exercise this functionality (Reference: testing guidelines)

• [ ] I have verified that all unit tests pass with the proposed changes

• [x] I have submitted a signed Contributor License Agreement (Reference: Contributor License Agreement instructions)

[jesschimein]

Filed as internal issue #USD-10690

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[jesschimein]

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