[release-branch.go1.17] 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 <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>
(cherry picked from commit b7529c3)
Reviewed-on: https://go-review.googlesource.com/c/go/+/362535
Trust: Bryan C. Mills <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>
Reviewed-by: Cuong Manh Le <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: cuonglm

src/cmd/go/testdata/script/mod_get_direct.txt

@@ -10,7 +10,7 @@ env GO111MODULE=on
 env GOPROXY=direct
 env GOSUMDB=off
 
-go list -m cloud.google.com/go@master
+go list -m cloud.google.com/go@main
 ! stdout 'v0.0.0-'
 
 -- go.mod --