fix double copy

Why
===
* New lazy trees nix warns about double copies from using ./.

What changed
===
* Replace with recommended replacement

Test plan
===
* `nix build` works

Author: ryantm

toml-editor.nix

@@ -10,5 +10,9 @@ rustPlatform.buildRustPackage {
     lockFile = ./Cargo.lock;
   };
 
-  src = ./.;
+  src = builtins.path {
+    path = ./.;
+    name = "source";
+  };
+
 }