The problem was introduced by the recent filepath.Join change.

Fixes #14949

Change-Id: I7ee52f210e12bbb1369e308e584ddb2c7766e095
Reviewed-on: https://go-review.googlesource.com/23240
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>