fix(serve): content can be served from output_path

This fixes a bug introduced in #2258

The issue arose when `output_path` was relative. The request being
served would be canonicalized and this would be a string. So, for
example, if you were serving content from `public` the code
[right after](https://github.com/getzola/zola/blob/38199c125501e9ff0e700e96adaca72cc3f25d2b/src/cmd/serve.rs#L144-L147)
the canonicalization checking if
`root.starts_with(original_root)` would always return `false` since
an absolute path, `/some/path/to/content` would never start with a
string like `public`.

src/cmd/serve.rs

@@ -461,9 +461,9 @@ pub fn serve(
     let ws_address = format!("{}:{}", interface, ws_port.unwrap());
     let output_path = site.output_path.clone();
 
-    // output path is going to need to be moved later on, so clone it for the
-    // http closure to avoid contention.
-    let static_root = output_path.clone();
+    // static_root needs to be canonicalized because we do the same for the http server.
+    let static_root = std::fs::canonicalize(&output_path).unwrap();
+
     let broadcaster = {
         thread::spawn(move || {
             let addr = address.parse().unwrap();