Build(deps): 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](nix-rust/nix@v0.20.0...v0.21.0)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Cargo.lock

@@ -28,7 +28,7 @@ indicatif = "0.15.0"
 ipnetwork = "0.18.0"
 lazy_static = "1.4.0"
 log = { version = "0.4.14", features = ["serde", "std"] }
-nix = "0.20.0"
+nix = "0.21.0"
 parking_lot = "0.11.1"
 proc-mounts = "0.2.4"
 rand = "0.8.3"