Update nix requirement from 0.30 to 0.31

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](https://github.com/nix-rust/nix/compare/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] <[email protected]>

Cargo.toml

@@ -68,7 +68,7 @@ windows-service = "0.8"
 
 [target.'cfg(unix)'.dependencies]
 daemonize = "0.5"
-nix = { version = "0.30", default-features = false, features = [
+nix = { version = "0.31", default-features = false, features = [
     "fs",
     "socket",
     "uio",