cmd/go: make `go mod download` with no arguments leave go.sum alone [1.16 backport]

[gopherbot]

@jayconrod requested issue #45332 to be considered for backport to the next 1.16 minor release.

@gopherbot Please backport to 1.16. We plan to revert to the 1.15 behavior.

[bcmills]

We should make sure this lands sooner rather than later, so that folks who run later in the upgrade cycle don't experience as much churn adapting to 1.16.

[gopherbot]

Change https://golang.org/cl/321892 mentions this issue: [release-branch.go1.16] cmd/go: in 'go mod download' without args, don't save module zip sums

[mknyszek]

@bcmills This looks OK to me as a backport, since it's fixing behavior that (IIUC) users can't easily work around (or if they can, it's a significant burden to have to make users use a different method for Go 1.15 and earlier vs. Go 1.16 and later), and that has a negative impact on the UX of the modules. The change in behavior was introduced in Go 1.16. Does that sound about right?

[bcmills]

Yep, that sounds accurate.

[gopherbot]

Closed by merging dbf69b7 to release-branch.go1.16.