Bump nix from 0.14.1 to 0.15.0 (#5488)

Bumps [nix](https://github.com/nix-rust/nix) from 0.14.1 to 0.15.0.
- [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>

netutil/Cargo.toml

@@ -12,7 +12,7 @@ edition = "2018"
 bincode = "1.1.4"
 clap = "2.33.0"
 log = "0.4.8"
-nix = "0.14.1"
+nix = "0.15.0"
 rand = "0.6.1"
 socket2 = "0.3.11"
 solana-logger = { path = "../logger", version = "0.18.0-pre1" }