This makes the rendered lock files more useful for users (they won't
install conda/mamba in a non-base environment) and will allow for the
creation of a metapackage for each release.
The pkg installer does not have any environment variables (like $PREFIX)
defined, but by inspection the other scripts that it executes set
the PREFIX as $2/{installer_name}. So we do that when $PREFIX is not
defined, which should cover all of the installers.
