In service of https://github.com/flutter/flutter/issues/143348

When invoking a package to transform an asset, we set `FLUTTER_BUILD_MODE` to the CLI name of the build mode being used. Inspired by https://github.com/flutter/flutter/issues/101077#issuecomment-1890379501:

> Do transformers know whether they get executed in debug or release mode? I kinda imagine that being useful. Ex: There's a transformer that optimizes the file size of images. Depending on the amount and size of the images, that could take a significant amount of time. Therefore, I might want to only execute it in release builds.

Note for the reviewer: the interesting part of this change can be found in the commit [set environment variable to build mode when running asset transformer…](https://github.com/flutter/flutter/pull/144752/commits/579912d470b6f6eb036fd6b23a128fa3942f6282). The rest of the change is updating call sites with a new argument.