internal/worker: measure disk usage for a new modules location

Otherwise, du fails as the location does not exist.

Change-Id: I849a263ae2c26f2c86cd546a15af79b2b0391daf
Reviewed-on: https://go-review.googlesource.com/c/pkgsite-metrics/+/476317
Run-TryBot: Zvonimir Pavlinovic <zpavlinovic@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/internal/worker/scan.go b/internal/worker/scan.go
index 1990282..e7ae2fe 100644
--- a/internal/worker/scan.go
+++ b/internal/worker/scan.go
@@ -72,7 +72,7 @@
 
 	logDiskUsage := func(msg string) {
 		log.Debugf(ctx, "sandbox disk usage %s clean:\n%s",
-			msg, diskUsage("/bundle/rootfs/root", "/bundle/rootfs/modules"))
+			msg, diskUsage(filepath.Join(sandboxRoot, "root"), modulesDir))
 	}
 
 	if insecure {