cmd: update x/mod to pull in CL 316113

Even though x/mod is now lazy, this has no immediate effect on the
size of cmd/go.sum: all of the existing dependencies are still pulled
in via the dependency on x/tools, which is itself not yet lazy (that's
CL 315570).

For #36460
Updates #36905

Change-Id: I7bce5fe2596a2d71e4df08f5d5f4cb8dcdb8512c
Reviewed-on: https://go-review.googlesource.com/c/go/+/316489


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>