cmd: update requirement on golang.org/x/mod

Commands run:
  go get golang.org/x/mod@master
  go mod vendor
  go mod tidy

This change pulls in the x/mod on master. Before it was pulled in on
the dev.cmdgo branch of x/mod, but now that the workspace changes have
been pulled into x/mod, we can change the requirement back on to the
main branch.

Change-Id: I296799a87ecc08ba17c0722e955f000426b105ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/345390


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>