CMake recognizes the CMAKE_PREFIX_PATH environment variable for some
things, and also the CMAKE_PREFIX_PATH cache variable for other things.
However, it does not relate these two things, i.e. if the environment
variable is set, CMake does not populate the cache variable in a
corresponding manner. Some package systems, such as Homebrew, set the
environment variable but not the cache variable. Furthermore, the
environment variable follows the system path separator, such as ':' on
POSIX and ';' on Windows, but the cache variable follows CMake's array
behavior, i.e. always ';' for a separator.
Closes#13242
0ae60f7236