databases/cockroach: Unbreak with go1.16

> Building for cockroach-2.0.7

...
go: cannot find main module, but found Gopkg.lock in /wrkdirs/usr/ports/databases/cockroach/work/cockroach-v2.0.7/src/github.com/cockroachdb/cockroach

to create a module there, run:
go mod init