Fix sitemap URL and last modified date formatting

Updated sitemap generation to use 'site.url' and format last modified date.

Author: retlehs

src/content/xml_sitemap.njk

@@ -8,8 +8,8 @@ layout: false
   {%- for page in collections.all -%}
     {%- if page.url != '' and page.data.sitemapIgnore != true -%}
       <url>
-        <loc>{{ env.siteUrl }}{{ page.url | url }}</loc>
-        {%- if page.date -%}<lastmod>{{ page.date }}</lastmod>{%- endif -%}
+        <loc>{{ site.url }}{{ page.url | url }}</loc>
+        {%- if page.date -%}<lastmod>{{ page.date.toISOString() }}</lastmod>{%- endif -%}
         {%- if page.data.changefreq -%}<changefreq>{{ page.data.changefreq }}</changefreq>{%- endif -%}
         {%- if page.data.priority -%}<priority>{{ page.data.priority }}</priority>{%- endif -%}
       </url>