clippy fixes: Box::new(_) of default value

From rust-lang:

> Why is this bad?
>
> First, it’s more complex, involving two calls instead of one. Second,
> Box::default() can be faster in certain cases.

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>

src/acme/plugin.rs

@@ -35,7 +35,7 @@ pub(crate) fn get_acme_plugin(
         }
         "standalone" => {
             // this one has no config
-            Box::new(StandaloneServer::default())
+            Box::<StandaloneServer>::default()
         }
         other => bail!("missing implementation for plugin type '{}'", other),
     }))