upgrade nix to 0.26.4

Summary:
I am upgrading nix to 0.29.0. To ensure each step produces clean CI
and is bisectable if something goes wrong, start by going from 0.25 to
0.26.4.

Reviewed By: zertosh

Differential Revision: D66275116

fbshipit-source-id: 63c87ff4f4c631061e5a1c721f52563dc90b7e2b

watchman/cli/Cargo.toml

@@ -22,13 +22,13 @@ tokio = { version = "1.41.0", features = ["full", "test-util", "tracing"] }
 watchman_client = { version = "0.9.0", path = "../rust/watchman_client" }
 
 [target.'cfg(target_os = "linux")'.dependencies]
-nix = "0.25"
+nix = "0.26.4"
 
 [target.'cfg(target_os = "macos")'.dependencies]
-nix = "0.25"
+nix = "0.26.4"
 
 [target.'cfg(unix)'.dependencies]
-nix = "0.25"
+nix = "0.26.4"
 
 [features]
 default = []