refactor(3p/nix): Apply clang-tidy's performance-* fixes

This applies the performance fixes listed here:

https://clang.llvm.org/extra/clang-tidy/checks/list.html

third_party/nix/src/libexpr/primops.hh

@@ -11,7 +11,7 @@ struct RegisterPrimOp {
   /* You can register a constant by passing an arity of 0. fun
      will get called during EvalState initialization, so there
      may be primops not yet added and builtins is not yet sorted. */
-  RegisterPrimOp(std::string name, size_t arity, PrimOpFun fun);
+  RegisterPrimOp(const std::string& name, size_t arity, PrimOpFun fun);
 };
 
 /* These primops are disabled without enableNativeCode, but plugins