[third-party/rust] 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.

Test Plan:
CI

```
arc rust-check //eden/... //apsw/usd/... //ai_productivity/plugboard/... //common/rust/... //watchman/...
```

Reviewed By: zertosh

Differential Revision: D66275116

fbshipit-source-id: 63c87ff4f4c631061e5a1c721f52563dc90b7e2b

antlir/antlir2/sendstream_parser/Cargo.toml

@@ -18,7 +18,7 @@ path = "src/lib.rs"
 bytes = { version = "1.6.0", features = ["serde"] }
 derive_more = { version = "1.0.0", features = ["full"] }
 hex = "0.4.3"
-nix = "0.25"
+nix = "0.26.4"
 nom = "7.1"
 serde = { version = "1.0.185", features = ["derive", "rc"], optional = true }
 thiserror = "1.0.64"