cmd/go: write changes to go.mod and go.sum after loading the command-line-arguments...

cmd/go: write changes to go.mod and go.sum after loading the command-line-arguments package

This entrypoint was missed in CL 349600, and the behavior happened not
to be covered by existing tests.

Fixes #52331.

Change-Id: Iccf12e8e633215abe4bfa1c3ca2fe3a8391b5ba5
Reviewed-on: https://go-review.googlesource.com/c/go/+/401536
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>