Revert "core: Previous commits didn't actually remove nix as dependency, so do that here"

This reverts commit cca3846f95, we need to
bring it back unfortunately.

Cargo.lock

@@ -1217,6 +1217,7 @@ dependencies = [
  "miette",
  "mimalloc",
  "mockall",
+ "nix 0.29.0",
  "pest",
  "pest_derive",
  "polling",

core/Cargo.toml

@@ -40,6 +40,7 @@ fallible-iterator = "0.3.0"
 hex = "0.4.3"
 libc = "0.2.155"
 log = "0.4.20"
+nix = { version = "0.29.0", features = ["fs"] }
 sieve-cache = "0.1.4"
 sqlite3-parser = { path = "../vendored/sqlite3-parser" }
 thiserror = "1.0.61"