BUILD_ROOT + BUILD_INSTALLR_ROOT vars

This information is interpretable in other ways, but requires unneeded string manipulation.

REZ_BUILD_INSTALL_PATH points to the installing-variant's build path, e.g. /path/to/pkg/vers/variant
and REZ_BUILD_PATH does the same:
/path/to/builds/pkg/build/variant

however if you need the unvarianted root for some reason during your build, it requires some hoops as mentioned, because REZ_PKGNAME_ROOT is not available yet for the package being built.
Would be nice to have a:
REZ_BUILD_INSTALL_ROOT = /path/to/pkg/vers
REZ_BUILD_ROOT = /path/to/builds/pkg/build

Start by tracing where REZ_BUILD_PATH and REZ_BUILD_INSTALL_PATH are defined and populated. Add the corresponding unvarianted root values, and verify that they resolve to the package/version paths described in the issue during a build.

Written by the indexing model from the issue text.