time: fix receiver for Time.IsDST method

Only methods that modify the time take pointer receivers;
IsDST does not modify it and therefore should not.

For #42102 and #46688.

Change-Id: I4721ef7f4d7572236ae6e4d99a459b9ffb11999e
Reviewed-on: https://go-review.googlesource.com/c/go/+/326789


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>