Update nix requirement from 0.25 to 0.26 (#605)

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](nix-rust/nix@v0.25.0...v0.26.1)

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

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

examples/hello_world_until/Cargo.toml

@@ -12,4 +12,4 @@ futures-util = "0.3.14"
 tokio = { version = "1.11.0", features = ["full"] }
 
 [target.'cfg(unix)'.dev-dependencies]
-nix = "0.25"
+nix = "0.26"