Currently podhelper.rb will always point plugin builds at the cached engine artifacts, even when using `--local-engine`. In most cases this is fine, since when the final build actually runs it will be using the engine bundled into the app build, which will be the correct local engine build. When trying to test a local engine build with API additions against a local plugin modified to use those additions to ensure that they are working as expected, however, compilation will fail, because the new APIs won't be present in the plugin build.

This fixes that for macOS, and adds a TODO for iOS (which is more complicated to fix due to the host vs target build distinction).

macOS portion of https://github.com/flutter/flutter/issues/132228