build(rust): vendor rnix parser crate

third_party/crates/rnix-0.14.0/shell.nix

@@ -0,0 +1,9 @@
+let
+  flake-compat = builtins.fetchTarball {
+    url = "https://github.com/edolstra/flake-compat/archive/99f1c2157fba4bfe6211a321fd0ee43199025dbf.tar.gz";
+    sha256 = "0x2jn3vrawwv9xp15674wjz9pixwjyj3j771izayl962zziivbx2";
+  };
+in
+(import flake-compat {
+  src = ./.;
+}).shellNix.default