Fix clippy warning

warning: called `skip(x).next()` on an iterator. This is more succinctly expressed by calling `nth(x)`
  --> build.rs:20:20
   |
20 |       let mut path = out_path
   |  ____________________^
21 | |       .ancestors()
22 | |       .skip(3)
23 | |       .next()
   | |_____________^
   |
   = note: `#[warn(clippy::iter_skip_next)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#iter_skip_next

build.rs

@@ -19,8 +19,7 @@ fn get_assets_dir() -> PathBuf{
 
     let mut path = out_path
       .ancestors()
-      .skip(3)
-      .next()
+      .nth(4)
       .unwrap()
       .to_owned();
     path.push("assets");