Rewrite latest_flutter_candidate.dart script as a shell script

[kenzieschmoll]

Since this is written in dart, on our bots we have to build flutter tool twice:

1. Clone flutter and build so that we can run the latest_flutter_candidate.dart script
2. Checkout the branch name output by latest_flutter_candidate.dart and rebuild flutter tool.

This is costing us several minutes of runtime on our Github Action jobs. If we rewrite latest_flutter_candidate.dart as a shell script, we do not need to clone and build flutter to run the script, and therefore we can check flutter out at the latest_flutter_candidate version directly and just build once.