Prefer .nth(n) over .skip(n).next().

Found by clippy.

src/test/run-pass-fulldeps/auxiliary/proc_macro_def.rs

@@ -19,7 +19,7 @@ use proc_macro::*;
 
 #[proc_macro_attribute]
 pub fn attr_tru(_attr: TokenStream, item: TokenStream) -> TokenStream {
-    let name = item.into_iter().skip(1).next().unwrap();
+    let name = item.into_iter().nth(1).unwrap();
     quote!(fn $name() -> bool { true })
 }
 

src/test/run-pass/command-before-exec.rs

@@ -24,7 +24,7 @@ use std::sync::Arc;
 use std::sync::atomic::{AtomicUsize, Ordering};
 
 fn main() {
-    if let Some(arg) = env::args().skip(1).next() {
+    if let Some(arg) = env::args().nth(1) {
         match &arg[..] {
             "test1" => println!("hello2"),
             "test2" => assert_eq!(env::var("FOO").unwrap(), "BAR"),