Account for go mod download in go1.17 not updating go.sum

[pmalek-sumo]

Because the behaviour of go mod download changes in go1.17 we need to account for that. For previous versions this will result in the same behaviour.

go mod download
When go mod download is invoked without arguments, it will no longer save sums for downloaded module content to go.sum. It may still make changes to go.mod and go.sum needed to load the build list. This is the same as the behavior in Go 1.15. To save sums for all modules, use go mod download all.

ref: https://tip.golang.org/doc/go1.17

[fangyi-zhou]

This seems a better solution than what I did in d6406b0

[jpkrohling]

Sorry, I ended up merging @fangyi-zhou's PR first, which caused a conflict with this. @pmalek-sumo, could you please rebase? I'll merge it right after that and perform a release.

[pmalek-sumo]

@jpkrohling Done, please recheck.