fix: hardcode last byte-unit version that uses Rust 2018

Author: DrPeterVanNostrand

fil-proofs-tooling/Cargo.toml

@@ -47,7 +47,8 @@ rayon = "1.3.0"
 flexi_logger = "0.16.1"
 typenum = "1.11.2"
 generic-array = "0.14.4"
-byte-unit = "4.0.9"
+# `byte-unit = 4.0.14` uses Rust edition 2021.
+byte-unit = "=4.0.13"
 fdlimit = "0.2.0"
 dialoguer = "0.8.0"
 structopt = "0.3.12"