remote-store: don't log raw stderr by default

For remote stores the log messages are already forwarded as structured
STDERR_RESULT messages so the old format is duplicate information.  But
still included with -vvv since it could be useful for debugging
problems.

    $ nix build -L /nix/store/nl71b2niws857ffiaggyrkjwgx9jjzc0-foo.drv --store ssh-ng://localhost
    Hello World!
    foo> Hello World!
    [1/0/1 built] building foo

Fixes #3556

src/libstore/remote-store.cc

@@ -779,8 +779,10 @@ std::exception_ptr RemoteStore::Connection::processStderr(Sink * sink, Source *
             return std::make_exception_ptr(Error(status, error));
         }
 
-        else if (msg == STDERR_NEXT)
-            printError(chomp(readString(from)));
+        else if (msg == STDERR_NEXT) {
+            string s = chomp(readString(from));
+            printMsg(lvlVomit, "stderr %s", s);
+        }
 
         else if (msg == STDERR_START_ACTIVITY) {
             auto act = readNum<ActivityId>(from);