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/nix-build/nix-build.cc

@@ -118,7 +118,7 @@ static void _main(int argc, char** argv) {
         lines.pop_front();
         inShebang = true;
         for (int i = 2; i < argc; ++i) {
-          savedArgs.push_back(argv[i]);
+          savedArgs.emplace_back(argv[i]);
         }
         args.clear();
         for (auto line : lines) {