bug#61987: 'current-guix' uses configure flags of Git checkout

[Winter]

Hi,

Given the following commands from a fresh checkout (after the keyring branch 
has been fetched and 'guix git authenticate' has been run):

./bootstrap
./configure --localstatedir=/var --sysconfdir=/hi/guix
make
./pre-inst-env guix system image -t iso9660 gnu/system/install.scm

Booting the resulting ISO will result in the installer running a Guix instance 
that has a sysconfdir of '/hi/guix', which can be checked by looking at the 
contents of '/run/current-system/profile/share/guile/site/3.0/guix/config.scm':

(define-public %sysconfidr "/hi/guix")

This is caused because the installer uses 'current-guix', and it seemingly does 
not reconfigure the build (likely due to the use of 'channel-build-system').

Thanks,
Winter