build: Fix known-good MoltenVK building

build: Fix known-good MoltenVK building

MoltenVK seems to have trouble building on the
current version of Vulkan-Headers.

Since Vulkan-Headers doesn't take very long to build and
changes often, this fix removes the reuse of the
Vulkan-Headers repo, whose version is provided by Vulkan-Tools
known-good.json, in the MoltenVK build.

It instead uses the version of Vulkan-Headers provided by
the MoltenVK known-good when building MoltenVK.

**Note:** Vulkan-Tools and other required repos are still
built with the version of Vulkan-Headers provided by
Vulkan-Tools known-good.json.

Change-Id: I1b11122cf0c6ba488acd53a35e4755f00e48376c