cqfd: fix test of empty build_cmd

When there is no build.command defined in ${cqfdrc}, we still want to be
able to run command with "cqfd run", and this is why we have to test
$build_cmd after assigning it from "cqfd run"

cqfd

@@ -610,16 +610,16 @@ done
 
 config_load $flavor
 
-if [ -z "$build_cmd" ]; then
-	die "No build.command defined in $cqfdrc !"
-fi
-
 if $has_alternate_command; then
 	build_cmd="$*"
 elif [ -n "$*" ]; then
 	build_cmd+=" $*"
 fi
 
+if [ -z "$build_cmd" ]; then
+	die "No build.command defined in ${cqfdrc}, and no command specified"
+fi
+
 docker_run "$build_cmd"
 
 if $has_to_release; then