Way to differentiate in CMake whether rez is building, build-installing, releasing

Some sort of environment-variable for differentiating whether rez is being run as rez-build, rez-build -i, rez-release, and perhaps rez-build -c -i, and/or possibly to hook on to the --prefix if specified, would be very useful to have access to from CMake's point of view when building a package for custom behavior when needed.

Start by tracing the entry points for rez-build, rez-build -i, rez-release, and rez-build -c -i, then review how a CMake package build receives environment and --prefix information. Done means defining and documenting a consistent way for CMake to distinguish these modes and adding coverage for the supported cases.

Written by the indexing model from the issue text.