cmd/go/internal/mvs: in Upgrade, pass upgrades to buildList as upgrades

This has no impact on the resulting build list, but provides clearer
diagnostics if reqs.Required returns an error for one of the upgraded
modules.

For #37438

Change-Id: I5cd8f72a9b7b9a0b185e1a728f46fefbd2f09b4a
Reviewed-on: https://go-review.googlesource.com/c/go/+/266897


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>