[workflow] Upgrade mono_repo to v6.4.1

[techouse]

This will resolve the Node 12 deprecation warning

Node.js 12 actions are deprecated. For more information see: https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/. Please update the following actions to use Node.js 16: actions/cache, actions/checkout, actions/checkout, actions/cache

Read more here.

[techouse]

Looks like Github still complains about save-state, however, this is the latest mono_repo at the moment

The save-state command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

So I reported the issue google/mono_repo.dart#421

[techouse]

So I reported the issue google/mono_repo.dart#421

Looks like it was fixed. I'll wait on v6.4.2 and then make another PR.