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/build-remote/build-remote.cc

@@ -184,7 +184,7 @@ static int _main(int argc, char* argv[]) {
 #if __APPLE__
         futimes(bestSlotLock.get(), NULL);
 #else
-        futimens(bestSlotLock.get(), NULL);
+        futimens(bestSlotLock.get(), nullptr);
 #endif
 
         lock = -1;