[release-branch.go1.15] cmd/go: fix mod_get_fallback test

Fixes #43797

Change-Id: I3d791d0ac9ce0b523c78c649aaf5e339a7f63b76
Reviewed-on: https://go-review.googlesource.com/c/go/+/284797


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
(cherry picked from commit be28e5ab)
Reviewed-on: https://go-review.googlesource.com/c/go/+/284799