ggml-vulkan/CMakeLists: add a check for SPIRV-Headers (llama/22009)

ggml-vulkan/CMakeLists: add a check for SPIRV-Headers (llama/22009)

* ci/run: set explicit SPIR-V Headers search path for macOS vulkan CI

For whatever reason, the files are under additional sub-path
`vulkan/` under the cmake directory, which does not match either
current LunarG macOS Vulkan SDK structure (`lib/cmake/SPIRV-Headers`),
nor what gets installed when you run the cmake build+install for
SPIRV-Headers itself on at least Linux (`share/cmake/SPIRV-Headers`).

This allows for SPIRV-Headers to be found, as currently the CI
runner's setup does not seem to include the relevant path in
list of search locations.

* ggml-vulkan/CMakeLists: add a check for SPIRV-Headers

This is installed by the project if it is built and installed.
Receiving an error during the configuration step is generally
preferred to receiving an error in the middle of a build.