nix-eval-jobs: print evaluation results using writeToStdout

Lix blocks SIGPIPE, and std::cout ignores EPIPE (actually it probably stashes
an error code in some flag that no one ever reads, with the same end result).
Consequently, nix-eval-jobs would waste resources by continuing to evaluate
even when the reader interested in the results has long gone away. Instead, use
writeToStdout, which throws an error on EPIPE, leading to the process
terminating as desired.

Reported-by: Winter <winter@winter.cafe>

Change-Id: I962c09bab582a8ed27dd41c01b1519876a6a6964

subprojects/nix-eval-jobs/src/constituents.cc

@@ -170,6 +170,6 @@ void rewriteAggregates(std::map<std::string, nix::JSON> &jobs,
             job["error"] = ss.str();
         }
 
-        std::cout << job.dump() << "\n" << std::flush;
+        nix::logger->writeToStdout(job.dump());
     }
 }

subprojects/nix-eval-jobs/src/nix-eval-jobs.cc

@@ -320,7 +320,7 @@ void collector(MyArgs &myArgs, Sync<State> &state_,
                 auto named = response.find("namedConstituents");
                 if (named == response.end() || named->empty()) {
                     response.erase("namedConstituents");
-                    std::cout << response.dump() << "\n" << std::flush;
+                    logger->writeToStdout(response.dump());
                 }
             }
 
@@ -421,14 +421,14 @@ int main(int argc, char **argv) {
                         state->jobs[e.a]["error"] = e.message();
                         state->jobs[e.b]["error"] = e.message();
 
-                        std::cout << state->jobs[e.a].dump() << "\n"
-                                  << state->jobs[e.b].dump() << "\n";
+                        logger->writeToStdout(state->jobs[e.a].dump());
+                        logger->writeToStdout(state->jobs[e.b].dump());
 
                         for (const auto &jobName : e.remainingAggregates) {
                             state->jobs[jobName]["error"] =
                                 "Skipping aggregate because of a dependency "
                                 "cycle";
-                            std::cout << state->jobs[jobName].dump() << "\n";
+                            logger->writeToStdout(state->jobs[jobName].dump());
                         }
                     },
                 },