Update nix requirement from 0.14 to 0.15

Updates the requirements on [nix](https://github.com/nix-rust/nix) to permit the latest version.
- [Release notes](https://github.com/nix-rust/nix/releases)
- [Changelog](https://github.com/nix-rust/nix/blob/master/CHANGELOG.md)
- [Commits](https://github.com/nix-rust/nix/commits)

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

acmed/Cargo.toml

@@ -30,4 +30,4 @@ serde = { version = "1.0", features = ["derive"] }
 toml = "0.5"
 
 [target.'cfg(unix)'.dependencies]
-nix = "0.14"
+nix = "0.15"