Otherwise, on systems for which syscall does not implement Getwd,
a lot of unnecessary files and directories get added to the testlog,
right up the root directory. This was causing tests on such systems
to fail to cache in practice.

Updates #22593

Change-Id: Ic8cb3450ea62aa0ca8eeb15754349f151cd76f85
Reviewed-on: https://go-review.googlesource.com/83455
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>