Update nix requirement from 0.26 to 0.28

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@v0.26.0...v0.28.0)

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

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

Cargo.toml

@@ -30,7 +30,7 @@ rustix = { version = "0.38", default-features = false, features = ["event", "fs"
 thiserror = "1.0.7"
 
 [target.'cfg(unix)'.dependencies]
-nix = { version = "0.26", default-features = false, features = ["signal"], optional = true }
+nix = { version = "0.28", default-features = false, features = ["signal"], optional = true }
 
 [dev-dependencies]
 futures = "0.3.5"