hugolib: Fix .Site.LastChange setting

If weights are added to the content front matter, the default sort order
can't be relied upon to set `.Site.LastChange`.

Fixes gohugoio#2909

hugolib/site.go

@@ -1341,7 +1341,8 @@ func readCollator(s *Site, results <-chan HandledResult, errs chan<- error) {
 func (s *Site) buildSiteMeta() (err error) {
 	defer s.timerStep("build Site meta")
 
-	if len(s.Pages) == 0 {
+	ln := len(s.Pages)
+	if ln == 0 {
 		return
 	}
 
@@ -1356,7 +1357,7 @@ func (s *Site) buildSiteMeta() (err error) {
 
 	s.assembleMenus()
 
-	s.Info.LastChange = s.Pages[0].Lastmod
+	s.Info.LastChange = s.Pages.ByLastmod()[ln-1].Lastmod
 
 	return
 }