[dashboard] do not blindly erase the local clone of flutter repo

[yjbanov]

Currently, on every run of the dashboard build, we rm -rf the flutter repo and clone it from scratch. This makes it hard to experiment and fix bugs in-place, as all local changes are destroyed prior to the build. Instead, when we see local changes (using git status) we should:

• not rm -rf the repo
• not pull changes from upstream
• put a big warning banner in the output log
• let all tests run, but still fail the build at the end
  • this way people can test locally but the dashboard box will be sensitive to bugs that put local changes in the flutter repo, such as flutter_01.log crash files

/cc @devoncarew @sethladd @mraleph

[github-actions]

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