Bump nix from 0.20.0 to 0.21.0

Bumps [nix](https://github.com/nix-rust/nix) from 0.20.0 to 0.21.0.
- [Release notes](https://github.com/nix-rust/nix/releases)
- [Changelog](https://github.com/nix-rust/nix/blob/master/CHANGELOG.md)
- [Commits](https://github.com/nix-rust/nix/compare/v0.20.0...v0.21.0)

Signed-off-by: dependabot[bot] <support@github.com>

Cargo.toml

@@ -80,7 +80,7 @@ wait-timeout = "0.2.0"
 tempfile = "3.2.0"
 
 [target.'cfg(unix)'.dev-dependencies]
-nix = "0.20.0"
+nix = "0.21.0"
 
 [build-dependencies]
 clap = { version = "2.33", optional = true }