guix-install.sh: Fix chk_require exiting without warning.

* etc/guix-install.sh (chk_require): Because of the set -e option, Bash was
exiting when the command `command -v "$c" &>/dev/null' was running on its
own. Instead, we now test the command and accumulate warning messages in a
single statement by chaining the command with a logical OR.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

etc/guix-install.sh

@@ -86,9 +86,7 @@ chk_require()
     _debug "--- [ $FUNCNAME ] ---"
 
     for c in ${cmds[@]}; do
-        command -v "$c" &>/dev/null
-        [ "$?" -eq "1" ] &&
-            warn+=("$c")
+        command -v "$c" &>/dev/null || warn+=("$c")
     done
 
     [ "${#warn}" -ne 0 ] &&