cmd/go: update goSum if necessary

During investigation into #74691, we found a case where the hash was
missing from the goSum global var.  This change updates DownloadZip to
update the goSum hash if necessary when both the zipfile and
ziphashfile are already present.

For #74691

Change-Id: I904b7265f667256d0c60b66503a7954b467c6454
Reviewed-on: https://go-review.googlesource.com/c/go/+/705215
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>

src/cmd/go/internal/modfetch/fetch.go

@@ -214,9 +214,12 @@ func DownloadZip(ctx context.Context, mod module.Version) (zipfile string, err e
 		}
 		ziphashfile := zipfile + "hash"
 
-		// Return without locking if the zip and ziphash files exist.
+		// Return early if the zip and ziphash files exist.
 		if _, err := os.Stat(zipfile); err == nil {
 			if _, err := os.Stat(ziphashfile); err == nil {
+				if !HaveSum(mod) {
+					checkMod(ctx, mod)
+				}
 				return zipfile, nil
 			}
 		}