https://golang.org/cl/179998 made it an error to run go mod download
without args and without a go.mod. Don't do that.
Fixesgolang/go#32488
Change-Id: Icd845feb8fecae69b6a8363438bb9ee546a310dc
Reviewed-on: https://go-review.googlesource.com/c/tools/+/181298
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>