Also print output of the flutter command to stdout when using flutter_build action

[olimberg]

It would be nice to also get the output of the flutter command when using the flutter_build action.
Had to revert to using the flutter action for now.

I would be open to provide a pull request, if you could point out, what needs to be done.

[dotdoom]

I think this request is mostly a dup of #34, except in #34 we want output on --verbose and you'd rather have it always enabled.

Would you like to take a look at PR #36 which implements logging on --verbose, and see if that fits the purpose?