Parse unknown timezones as UTC (#1047)

Without this change, parsing changelog entries such as
Tue Mar 28 07:25:39 WEST 2017 - someone@suse

would produce a Mar 27 date in the .spec file, because

TZ=CET perl -e 'use Date::Parse; print str2time("28 Mar 2017")'
1490652000

considers the local timezone and maps to

date -d @1490652000 -u -Iseconds
2017-03-27T22:00:00+00:00

This patch was done while working on reproducible builds for openSUSE.

changelog2spec

@@ -33,6 +33,7 @@ use Time::Zone;
 
 use strict;
 
+$ENV{TZ} = "UTC";
 my @wday = qw{Sun Mon Tue Wed Thu Fri Sat};
 my @mon = qw{Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec};