Amends prematurely submitted CL 173918. We now use file:/// URLs for
go1.13 and later and file:// URLs for go1.12 and earlier.
Fixesgolang/go#31675
Change-Id: I009c63a900bdfd091bf46def5cea5a0843639b47
Reviewed-on: https://go-review.googlesource.com/c/tools/+/173919
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>
