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/nixexpr.cc

@@ -239,7 +239,7 @@ void ExprVar::bindVars(const StaticEnv& env) {
         withLevel = level;
       }
     } else {
-      StaticEnv::Vars::const_iterator i = curEnv->vars.find(name);
+      auto i = curEnv->vars.find(name);
       if (i != curEnv->vars.end()) {
         fromWith = false;
         this->level = level;