The need for this use case appeared when attempting to install Guix on a truly
minimal image made with Buildroot, which lacked enough GNU components that I
had to extract a guix pack to /gnu before attempting installation, which would
then refuse to proceed because of the existing /gnu.
* etc/guix-install.sh: Document environment variables.
(sys_create_store) [GUIX_ALLOW_OVERWRITE]: Skip pre-existing installation
checks and output a warning. Extract the tarball directly to /.