Previously, the check came after guix_get_bin. There’s no need to fetch
the rather large release archive if there’s an existing installation, so
check first. Refactor the check into a function similar to other
preflight checks.
* etc/guix-install.sh: Check for existing installation before
downloading.
Change-Id: I5506fb1cacdc88bd6355e8dfa1f690acf7886c1f
Signed-off-by: Ludovic Courtès <ludo@gnu.org>