Allow engine_v2 recipes to fetch a build configuration from monorepo repository

[whesse]

As with the engine_v2 coordinator recipe engine_v2 when it runs a flutter/engine build, I would like
the coordinator to download the build properties for the monorepo builds from a .json configuration
file in the monorepo repository.

This can be done by adding flexibility to the build properties download step, so it checks out the monorepo repository
instead of the engine repository. Exactly the same check as is used in

https://flutter.googlesource.com/recipes/+/refs/heads/main/recipes/engine_v2/builder.py#207

can be added when downloading the config, to checkout monorepo instead of flutter/engine, and download from a path in that repo.

[keyonghan]

Has this been implemented?

[flutter-triage-bot]

This issue is assigned to @whesse but has had no recent status updates. Please consider unassigning this issue if it is not going to be addressed in the near future. This allows people to have a clearer picture of what work is actually planned. Thanks!

[flutter-triage-bot]

This issue was assigned to @whesse but has had no status updates in a long time. To remove any ambiguity about whether the issue is being worked on, the assignee was removed.