This avoids a race condition with go1.go wanting to examine files in
the current directory with filepath.Walk(".", walkFn).
Fixes#10497.
Change-Id: I2159f40a08d1a768195dbb7ea3c27e38cf9740bb
Reviewed-on: https://go-review.googlesource.com/9110
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>