To build the packages you will need docker installed.
make docker_build
Since tfenv is installing terraform versions into /usr/lib/tfenv, it needs root permissions to install new terraform versions.
So installing a new version requires sudo permissions. Using a new version does not.
sudo tfenv install 0.12.28
For this to change tfenv needs to be changed so the directory it stores downloaded versions in is under the users own home directory and not a system directory.