internal/modindex: parse changed time in local time zone

The os package will return ModTime values in the local time zone.
We have to parse the time stamp in the local time zone so that
comparisons are done correctly.

Fixes golang/go#69893
Fixes golang/go#69894

Change-Id: I93631cdc64dda8fdfcdf668e10e0ec939ef1461d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/624615
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Peter Weinberger <pjw@google.com>

Author: ianlancetaylor

internal/modindex/index.go

@@ -145,7 +145,7 @@ func readIndexFrom(cd Abspath, bx io.Reader) (*Index, error) {
 	}
 	// TODO(pjw): need to check that this is the expected cachedir
 	// so the tag should be passed in to this function
-	ans.Changed, err = time.Parse(time.DateTime, b.Text())
+	ans.Changed, err = time.ParseInLocation(time.DateTime, b.Text(), time.Local)
 	if err != nil {
 		return nil, err
 	}