Ignore clippy::needless_pass_by_ref_mut warning

```
warning: this argument is a mutable reference, but not used mutably
  --> src/experimental/env.rs:38:15
   |
38 |         args: &mut &'a ArgsBytes<BUF_SIZE>,
   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&&'a ArgsBytes<BUF_SIZE>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut
   = note: `#[warn(clippy::needless_pass_by_ref_mut)]` on by default
```

Author: taiki-e

src/experimental/env.rs

@@ -34,6 +34,7 @@ mod sys {
 
     const NUL: u8 = b'\0';
 
+    #[allow(clippy::needless_pass_by_ref_mut)]
     fn next_from_cmdline<'a, const BUF_SIZE: usize>(
         args: &mut &'a ArgsBytes<BUF_SIZE>,
     ) -> Option<&'a [u8]> {