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/export-import.cc

@@ -11,7 +11,7 @@ struct HashAndWriteSink : Sink {
   HashSink hashSink;
   explicit HashAndWriteSink(Sink& writeSink)
       : writeSink(writeSink), hashSink(htSHA256) {}
-  virtual void operator()(const unsigned char* data, size_t len) {
+  void operator()(const unsigned char* data, size_t len) override {
     writeSink(data, len);
     hashSink(data, len);
   }