ci/baremetal: fix logic for retrying boot when it failed

Contrary to what the original commit said, this is actually still used
(see .gitlab-ci/bare-metal/poe-powered.sh:205), and the boot retry logic
has been broken ever since, exacerbating the rpi farm boot problems.

Fixes: 97b2afa16a ("ci/bare-metal: Drop the 2 vs 1 exit code from poe_run.")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30340>

.gitlab-ci/bare-metal/poe_run.py

@@ -64,7 +64,7 @@ class PoERun:
         if not boot_detected:
             self.print_error(
                 "Something wrong; couldn't detect the boot start up sequence")
-            return 1
+            return 2
 
         self.logger.create_job_phase("test")
         for line in self.ser.lines(timeout=self.test_timeout, phase="test"):