Fix regression with site.IsServer when not running a server

Fixes #11006

testscripts/commands/hugo.txt

@@ -5,12 +5,13 @@ stdout 'Pages.*|1'
 stdout 'Total in'
 checkfile public/index.html
 checkfile public/p1/index.html
+grep 'IsServer: false' public/index.html
 
 -- hugo.toml --
 baseURL = "http://example.org/"
 disableKinds = ["RSS", "sitemap", "robotsTXT", "404", "taxonomy", "term"]
 -- layouts/index.html --
-Home.
+Home|IsServer: {{ .Site.IsServer }}|
 -- layouts/_default/single.html --
 Title: {{ .Title }}
 -- content/p1.md --