build-self: Don't clobber the output port.

The newline is meant to follow the spinner's traces so it must go to the
error port as well.

* build-aux/build-self.scm (build): Send newline to the error port.

build-aux/build-self.scm

@@ -420,7 +420,7 @@ files."
            (error "build program failed" (list build status)))
           ((? derivation-path? drv)
            (mbegin %store-monad
-             (return (newline (current-output-port)))
+             (return (newline (current-error-port)))
              ((store-lift add-temp-root) drv)
              (return (read-derivation-from-file drv))))
           ("#f"