cmd/go: 'go get' does not add missing hash to go.sum when ziphash file missing from cache [1.16 backport]

[gopherbot]

@jayconrod requested issue #44749 to be considered for backport to the next 1.16 minor release.

@gopherbot Please backport to 1.16. It's a pre-existing issue, but it's much more likely to be observed because of the switch to -mod=readonly by default.

[gopherbot]

Change https://golang.org/cl/298851 mentions this issue: [release-branch.go1.16] cmd/go/internal/modfetch: detect and recover from missing ziphash file

[dmitshur]

Approving per discussion in a release meeting. This backport applies to both 1.16 (this issue) and 1.15 (#44872).

[gopherbot]

Closed by merging 33fb479 to release-branch.go1.16.