Use a new filename for each invocation so invocations run
concurrently don't overwrite the same file.
Change-Id: I6a6a696478b596a4819f41b3ac738263d41bbabf
Reviewed-on: https://go-review.googlesource.com/c/go/+/699017 Reviewed-by: Michael Matloob <matloob@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Ian Lance Taylor <iant@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>