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/pathlocks.cc

@@ -97,7 +97,7 @@ bool PathLocks::lockPaths(const PathSet& paths, const string& waitMsg,
 
     AutoCloseFD fd;
 
-    while (1) {
+    while (true) {
       /* Open/create the lock file. */
       fd = openLockFile(lockPath, true);
 
@@ -136,7 +136,7 @@ bool PathLocks::lockPaths(const PathSet& paths, const string& waitMsg,
     }
 
     /* Use borrow so that the descriptor isn't closed. */
-    fds.push_back(FDPair(fd.release(), lockPath));
+    fds.emplace_back(fd.release(), lockPath);
   }
 
   return true;