Variant agnostic build steps

Is there an existing way of executing project-level (i.e. variant agnostic) build steps for packages with variants? Our packages contain a target to build Sphinx docs which are the same across all variants, so ideally we would like to only build them once.

Currently Rez treats each variant as a separate cmake project, so `PROJECT_BINARY_DIR` points to the variant's root directory, not the project's. I had a look through the cmake env vars and variables, but couldn't find anything that represented the project-level build directory. I can work around this with some path hackery, but it seems error-prone.

Thanks!

Start by reading Rez's CMake integration and tracing how PROJECT_BINARY_DIR is set for each package variant. Determine whether a project-level build directory is already exposed through CMake variables or environment variables; done means providing a supported way to run the Sphinx documentation target once for all variants.

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