build(rust): vendor rnix parser crate

third_party/crates/rnix-0.14.0/default.nix

@@ -0,0 +1,9 @@
+let
+  flake-compat = builtins.fetchTarball {
+    url = "https://github.com/edolstra/flake-compat/archive/12c64ca55c1014cdc1b16ed5a804aa8576601ff2.tar.gz";
+    sha256 = "0jm6nzb83wa6ai17ly9fzpqc40wg1viib8klq8lby54agpl213w5";
+  };
+in
+(import flake-compat {
+  src = ./.;
+}).defaultNix.default