cmd/mbf: expose target load average

There are often good reason to override this.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/mbf/main.go

@@ -210,6 +210,10 @@ func main() {
 		&cm.attr.Jobs,
 		"jobs", command.IntFlag(0),
 		"Preferred number of jobs to run, when applicable",
+	).Flag(
+		&cm.attr.Load,
+		"load-average", command.IntFlag(0),
+		"Preferred load average number, when applicable",
 	).Flag(
 		&cm.base,
 		"d", command.StringFlag("$MBF_CACHE_DIR"),