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/nix-store/nix-store.cc

@@ -28,6 +28,9 @@ using namespace nix;
 using std::cin;
 using std::cout;
 
+// TODO(tazjin): clang-tidy's performance lints don't like this, but
+// the automatic fixes fail (it seems that some of the ops want to own
+// the args for whatever reason)
 using Operation = void (*)(Strings, Strings);
 
 static Path gcRoot;