cmd/mbf: cache dir via environment

This is much less cumbersome than dragging the flag around all the time.

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

cmd/mbf/main.go

@@ -59,6 +59,11 @@ func main() {
 	c := command.New(os.Stderr, log.Printf, "mbf", func([]string) (err error) {
 		msg.SwapVerbose(!flagQuiet)
 
+		flagBase = os.ExpandEnv(flagBase)
+		if flagBase == "" {
+			flagBase = "cache"
+		}
+
 		var base *check.Absolute
 		if flagBase, err = filepath.Abs(flagBase); err != nil {
 			return
@@ -85,7 +90,7 @@ func main() {
 		"Maximum number of dependencies to cure at any given time",
 	).Flag(
 		&flagBase,
-		"d", command.StringFlag("cache"),
+		"d", command.StringFlag("$MBF_CACHE_DIR"),
 		"Directory to store cured artifacts",
 	).Flag(
 		&flagTShift,