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

@@ -45,7 +45,7 @@ libc = "0.2"
 log = { version = "0.4", features = ["max_level_trace", "release_max_level_debug"] }
 lz4-sys = "1.9"
 memmap2 = { version = "0.9", optional = true }
-nix = "0.26"
+nix = "0.28"
 num-derive = "0.4"
 num-traits = "0.2"
 parking_lot = "0.12"