add explicit paths to the flutter recipies

Previously we had the wrong path to the recipes.py script and an incorrect argument list.

@abarth

@abarth @chinmaygarde

This uses the fancy new x-repository support:
https://github.com/luci/recipes-py/blob/master/doc/cross_repo.md

I'm not at all certain that we're going to use Chromium's
Infra for flutter, but I'm exploring and since this works
I figure I should check it in.

@abarth