Fix render breakage introduced in #665 (#685)

A bare bones setup of two pages and two posts gets broken because of a
template change missed in #665

Fatal error message in readable form:
```
ERROR render:
failed to render pages:
render of "home" failed: "/themes/ananke/layouts/index.html:48:23":
execute of template failed:
template:
index.html:48:23:
executing "main" at <.Site.GetPage>: can't evaluate field Site in type string
```

Steps to reproduce:

```shell
hugo new site quickstart
cd quickstart
git init
git submodule add https://github.com/theNewDynamic/gohugo-theme-ananke.git themes/ananke
echo "theme = 'ananke'" >> hugo.toml
echo -e "+++\ntitle='Home'\n+++\n\nMain page" > content/_index.md
echo Foo. > content/page1.md
echo Bar > content/page2.md
mkdir -p content/post
echo "First post." > content/post/post1.md
echo "Second post." > content/post/post2.md
hugo server
Watching for changes in .../quickstart/{archetypes,assets,content,data,i18n,layouts,static,themes}
Watching for config changes in .../quickstart/hugo.toml, ...quickstart/themes/ananke/config.yaml
Start building sites …
hugo v0.125.3-474c4c02212cf97712c6fbf4159c68822ea6e078+extended darwin/amd64 BuildDate=2024-04-22T17:18:35Z VendorInfo=brew

Built in 67 ms
Error: error building site: render: failed to render pages: render of "home" failed: ".../quickstart/themes/ananke/layouts/index.html:48:23": execute of template failed: template: index.html:48:23: executing "main" at <.Site.GetPage>: can't evaluate field Site in type string
```