cmd/go: fix mod_get_direct
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 <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Run-TryBot: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

CAFxX/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,golang/go,nawawi/go,nawawi/go,golang/go,golang/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go