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/ssh.hh

@@ -25,7 +25,7 @@ class SSHMaster {
   void addCommonSSHOpts(Strings& args);
 
  public:
-  SSHMaster(const std::string& host, const std::string& keyFile, bool useMaster,
+  SSHMaster(const std::string& host, std::string keyFile, bool useMaster,
             bool compress, int logFD = -1);
 
   struct Connection {