refactor(tvix/nix-compat/wire): simplify write_bytes a bit

Use the same EMPTY_BYTES trick from BytesWriter to write out the
padding, rather than allocating a Vec.

Change-Id: Ifb4ba1b45b7388adbc135fc8e46fd3d3cedd30aa
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11375
Reviewed-by: picnoir picnoir <picnoir@alternativebit.fr>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/nix-compat/src/wire/bytes_writer.rs

@@ -3,12 +3,11 @@ use std::task::{ready, Poll};
 
 use tokio::io::AsyncWrite;
 
+use super::bytes::EMPTY_BYTES;
+
 /// The length of the size field, in bytes is always 8.
 const LEN_SIZE: usize = 8;
 
-/// 8 null bytes, used to write out padding.
-const EMPTY_BYTES: &[u8; 8] = &[0u8; 8];
-
 pin_project! {
     /// Writes a "bytes wire packet" to the underlying writer.
     /// The format is the same as in [crate::wire::bytes::write_bytes],