Currently the only way to do this is to do a normal build, then replace the library/framework with the locally built version (and remember that your build is no longer actually using the hash that .last_engine_version, and thus the build output, claims). Updating Flutter will stomp the replacement.

And you have to also remember to change
https://github.com/google/flutter-desktop-embedding/blob/master/tools/build_flutter_assets#L26
and undo it when unwinding.

There should be something along the lines of .flutter_location_config that both update_flutter_engine callers and build_flutter_assets will check against that will cause a local build to be copied in and used instead, such that correctly enabling/disabling it is as simple as moving a file. (Or, if we switch to GN, a GN configuration option might work instead.)