cmd/go: fix mod_get_direct

https://github.com/googleapis/google-cloud-go has changed the default
branch from master to main, causing mod_get_direct failed on longtest.

Change-Id: I8fe0356b2ff532d1fdedbcb1e1832d7335babaa0
Reviewed-on: https://go-review.googlesource.com/c/go/+/361965


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>