Reland of https://github.com/flutter/flutter/pull/133719. Updates [the string comparison in flavor_test.dart](https://github.com/flutter/flutter/pull/134060/files#diff-53383b32b975bfed6875306dfb98911cad077a5251ca0591c5b0e125fb4a0f05R39) to use `path.join` to build the path string so that the generated path is correct for both Linux and Windows hosts.

Fixes https://github.com/flutter/flutter/issues/133713

I've tested this on a Windows host targeting a physical Android device.