JSONLogger: Log to a file descriptor instead of another Logger

Logging to another Logger was kind of nonsensical - it was really just
an easy way to get it to write its output to stderr, but that only
works if the underlying logger writes to stderr.

This change is needed to make it easy to log JSON output somewhere
else (like a file or socket).

src/libstore/unix/build/local-derivation-goal.cc

@@ -2225,7 +2225,7 @@ void LocalDerivationGoal::runChild()
         /* Execute the program.  This should not return. */
         if (drv->isBuiltin()) {
             try {
-                logger = makeJSONLogger(*logger);
+                logger = makeJSONLogger(STDERR_FILENO);
 
                 std::map<std::string, Path> outputs;
                 for (auto & e : drv->outputs)