time: propagate "," separator for fractional seconds into Format

In CL 300996 that fixed issue #6189, we made Parse recognize
"," as a separator for fractional seconds.
However, we didn't modify Format to propagate the separator
verbatim from Parse. Without this change, we break prior
functionality that relied on a comma being used in Format.

Fixes #48037

Change-Id: I6565a25e8657ca3747a58b25acba58f27cdcddc0
Reviewed-on: https://go-review.googlesource.com/c/go/+/345438


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Trust: Cherry Mui <cherryyz@google.com>