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/legacy-ssh-store.cc

@@ -262,7 +262,7 @@ static RegisterStoreImplementation regStore(
     [](const std::string& uri,
        const Store::Params& params) -> std::shared_ptr<Store> {
       if (std::string(uri, 0, uriScheme.size()) != uriScheme) {
-        return 0;
+        return nullptr;
       }
       return std::make_shared<LegacySSHStore>(
           std::string(uri, uriScheme.size()), params);