Update nix requirement from >= 0.18, < 0.21 to >= 0.18, < 0.22

Updates the requirements on [nix](https://github.com/nix-rust/nix) to permit the latest version.
- [Release notes](https://github.com/nix-rust/nix/releases)
- [Changelog](https://github.com/nix-rust/nix/blob/master/CHANGELOG.md)
- [Commits](nix-rust/nix@v0.18.0...v0.21.0)

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

Author: dependabot[bot]

Cargo.toml

@@ -16,7 +16,7 @@ documentation = "http://docs.rs/openat-ext"
 [dependencies]
 openat = "0.1.15"
 libc = "^0.2.94"
-nix = ">= 0.18, < 0.21"
+nix = ">= 0.18, < 0.22"
 rand = "0.8.3"
 
 [dev-dependencies]