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/libstore/misc.cc

@@ -18,7 +18,7 @@ void Store::computeFSClosure(const PathSet& startPaths, PathSet& paths_,
     std::exception_ptr exc;
   };
 
-  Sync<State> state_(State{0, paths_, 0});
+  Sync<State> state_(State{0, paths_, nullptr});
 
   std::function<void(const Path&)> enqueue;