This initializes the environment with the following environment
variables set to empty, so that they are cleared when the environment is
activated and interference from other installations (e.g. PothosSDR) is
avoided:
GR_PREFIX
GRC_BLOCKS_PATH
UHD_PKG_PATH
VOLK_PREFIX
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.
