Add dart to the $PATH

[pavanpodila]

It would be good to put the Dart binaries on $PATH

[subosito]

Hi @pavanpodila, thanks for the suggestion, we can add this later when enough demand for this. For now, you can do:

- run: echo ::add-path::${FLUTTER_HOME}/bin/cache/dart-sdk/bin
- run: dart --version

[shouichi]

We need this feature too.

Background: We run a mock server written in dart for flutter drive.