build(deps): update nix requirement

Updates the requirements on [nix](https://github.com/nix-rust/nix) to permit the latest version.

Updates `nix` to 0.31.3
- [Changelog](https://github.com/nix-rust/nix/blob/master/CHANGELOG.md)
- [Commits](nix-rust/nix@v0.30.0...v0.31.3)

---
updated-dependencies:
- dependency-name: nix
  dependency-version: 0.31.3
  dependency-type: direct:production
  dependency-group: vhost
...

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

Author: dependabot[bot]

vhost-user-backend/Cargo.toml

@@ -27,7 +27,7 @@ vm-memory = { workspace = true, features = ["backend-mmap", "backend-atomic", "b
 vmm-sys-util = { workspace = true }
 
 [dev-dependencies]
-nix = { version = "0.30", features = ["fs"] }
+nix = { version = "0.31", features = ["fs"] }
 uuid = { version = "1.8.0", features=["v4"] }
 vhost = { path = "../vhost", version = "0.17.0", features = ["test-utils", "vhost-user-frontend", "vhost-user-backend"] }
 vm-memory = { workspace = true, features = ["backend-mmap", "backend-atomic"] }