maintner: set GitHubComment.Updated to UTC when cmut.Updated is non-nil
maintner: set GitHubComment.Updated to UTC when cmut.Updated is non-nil

This change fixes a copy-paste mistake from CL 38137, where the wrong
time variable was set to UTC when cmut.Updated is non-nil.

Change-Id: I4dc62abf9b70b52b700e3b6b688fa8adbc502d98
Reviewed-on: https://go-review.googlesource.com/111645
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build