dev: add dart sdk to path

Adding Flutter bin to the path doesn't expose the dart SDK which many people might be interested in using. For example, dartdoc for documentation generation.

src/installer.ts

@@ -62,6 +62,7 @@ export async function getFlutter(
 
   core.exportVariable('FLUTTER_HOME', toolPath);
   core.addPath(path.join(toolPath, 'bin'));
+  core.addPath(path.join(toolPath, 'bin/cache/dart-sdk'));
 }
 
 function osName(): string {