pkg/time: use UTC time zone for Unix and Parse

Change-Id: Ia38856c40bfca70620c7783d41ea3936a020b745
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2726
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/pkg/time/time_test.go b/pkg/time/time_test.go
index 0386c66..7646ee6 100644
--- a/pkg/time/time_test.go
+++ b/pkg/time/time_test.go
@@ -102,8 +102,8 @@
 		nano int64
 		want string
 	}{
-		{0, 0, "1970-01-01T01:00:00+01:00"},
-		{1500000000, 123456, "2017-07-14T04:40:00.000123456+02:00"},
+		{0, 0, "1970-01-01T00:00:00Z"},
+		{1500000000, 123456, "2017-07-14T02:40:00.000123456Z"},
 	}
 
 	for _, tc := range valid {