build(deps): update nix requirement from 0.25 to 0.26

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>

crates/vhost-user-backend/Cargo.toml

@@ -18,7 +18,7 @@ vm-memory = { version = "0.10.0", features = ["backend-mmap", "backend-atomic"]
 vmm-sys-util = "0.11.0"
 
 [dev-dependencies]
-nix = "0.25"
+nix = "0.26"
 vhost = { path = "../vhost", version = "0.6", features = ["vhost-user-master", "vhost-user-slave"] }
 vm-memory = { version = "0.10.0", features = ["backend-mmap", "backend-atomic", "backend-bitmap"] }
 tempfile = "3.2.0"