show hydra job count in trigger step

previously we immediately triggered all jobs, now we no longer do.
showing the total count at least somewhere is nice to have a rough
indication of how much longer a build may still need to run.

buildbot_nix/__init__.py

@@ -71,6 +71,7 @@ class BuildTrigger(steps.BuildStep):
         self._result_list = []
         self.ended = False
         self.waitForFinishDeferred = None
+        self.description = f"building {len(jobs)} hydra jobs"
         super().__init__(**kwargs)
 
     def interrupt(self, reason):