tools/check: `pigeon` runs old/cached version of Pigeon after Pigeon dependency is upgraded

[chrisbobbe]

Part of #621 was to upgrade Pigeon, from 17.2.0 to 18.0.0. After I did that, tools/check pigeon --all-files on my machine was running the old Pigeon, while in CI the newer Pigeon was running, as we want it to, and flagging the need to update generated files. I found a workaround: https://github.com/zulip/zulip-flutter/pull/621#issuecomment-2040764190 but it would be nice not to require a manual step.

Can we make it so when we change the version of the Pigeon dependency, the next run of tools/check pigeon --all-files uses the updated version?