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.cc

@@ -1,12 +1,14 @@
 #include "ssh.hh"
 
+#include <utility>
+
 namespace nix {
 
-SSHMaster::SSHMaster(const std::string& host, const std::string& keyFile,
+SSHMaster::SSHMaster(const std::string& host, std::string keyFile,
                      bool useMaster, bool compress, int logFD)
     : host(host),
       fakeSSH(host == "localhost"),
-      keyFile(keyFile),
+      keyFile(std::move(keyFile)),
       useMaster(useMaster && !fakeSSH),
       compress(compress),
       logFD(logFD) {