[PATCH] tpl/tplimpl: Fix incorrect lastBuildDate

cherry-pick from gohugoio/hugo@87bf2b9

Co-authored-by: Joe Mooring <joe@mooring.com>

Author: 2 people

layouts/_default/rss.xml

@@ -60,7 +60,7 @@
     <managingEditor>{{.}}{{ with $authorName }} ({{ . }}){{ end }}</managingEditor>{{ end }}{{ with $authorEmail }}
     <webMaster>{{ . }}{{ with $authorName }} ({{ . }}){{ end }}</webMaster>{{ end }}{{ with site.Copyright }}
     <copyright>{{ . }}</copyright>{{ end }}{{ if not .Date.IsZero }}
-    <lastBuildDate>{{ .Date.Format "Mon, 02 Jan 2006 15:04:05 -0700" | safeHTML }}</lastBuildDate>{{ end }}
+    <lastBuildDate>{{ (index $pages.ByLastmod.Reverse 0).Lastmod.Format "Mon, 02 Jan 2006 15:04:05 -0700" | safeHTML }}</lastBuildDate>{{ end }}
     {{- with .OutputFormats.Get "RSS" }}
     {{ printf "<atom:link href=%q rel=\"self\" type=%q />" .Permalink .MediaType | safeHTML }}
     {{- end }}