build(deps): update nix requirement from >=0.28.0, <0.31 to >=0.28.0, <0.32

Updates the requirements on [nix](https://github.com/nix-rust/nix) to permit the latest version.
- [Changelog](https://github.com/nix-rust/nix/blob/master/CHANGELOG.md)
- [Commits](nix-rust/nix@v0.28.0...v0.31.1)

---
updated-dependencies:
- dependency-name: nix
  dependency-version: 0.31.1
  dependency-type: direct:production
...

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

Author: dependabot[bot]

Cargo.toml

@@ -29,7 +29,7 @@ serialize = ["serde", "serde_json"]
 
 [dependencies]
 libc = "0.2.155"
-nix = { version = ">=0.28.0, <0.31", default-features = false, features = [ "signal", "user" ] }
+nix = { version = ">=0.28.0, <0.32", default-features = false, features = [ "signal", "user" ] }
 unit-prefix = "0.5"
 sysctl = ">=0.5.0, < 0.8"
 serde = { version="1.0.113", features = ["derive"], optional=true }