scripts: Only append "run journalctl" message if using journal

For the `ex container` case we don't log to the journal by default,
so this message was misleading.

Closes: #1181
Approved by: jlebon

src/libpriv/rpmostree-scripts.c

@@ -323,7 +323,8 @@ run_script_in_bwrap_container (int rootfs_fd,
 
   if (!rpmostree_bwrap_run (bwrap, cancellable, error))
     {
-      if (error)
+      /* If errors go to the journal, help the user/admin find them there */
+      if (error && rpmostree_stdout_is_journal ())
         {
           g_assert (*error);
           g_autofree char *errmsg = (*error)->message;