flutter/packages .cirrus config fetches directly from latest master and uses pub global activate to install flutter_plugin_tools. A build that succeeds at one point in time can fail next time at the same commit simply by fetching a different version of Flutter or plugin tools (example build breakage).

Culprit: https://github.com/flutter/packages/blob/85135ddad047d82aa5025496410a41b031c4c5df/.cirrus.yml#L7-L11