Update nix requirement from ^0.30.0 to ^0.31.0

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@v0.30.0...v0.31.0)

---
updated-dependencies:
- dependency-name: nix
  dependency-version: 0.31.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

Cargo.toml

@@ -31,7 +31,7 @@ maintenance = { status = "passively-maintained" }
 bindgen = { version = "^0.72", optional = true }
 cc = "^1.1.6"
 pkg-config = "^0.3.30"
-nix = { version = "^0.30.0", default-features = false, features = ["fs"] }
+nix = { version = "^0.31.0", default-features = false, features = ["fs"] }
 
 [lib]
 crate-type = ["lib", "staticlib"]