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

This applies the modernization fixes listed here:

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

The 'modernize-use-trailing-return-type' fix was excluded due to my
personal preference (more specifically, I think the 'auto' keyword is
misleading in that position).

third_party/nix/src/libexpr/get-drvs.hh

@@ -33,7 +33,7 @@ struct DrvInfo {
   string attrPath; /* path towards the derivation */
 
   DrvInfo(EvalState& state) : state(&state){};
-  DrvInfo(EvalState& state, const string& attrPath, Bindings* attrs);
+  DrvInfo(EvalState& state, string attrPath, Bindings* attrs);
   DrvInfo(EvalState& state, ref<Store> store,
           const std::string& drvPathWithOutputs);