(PUP-2816) Default forge override in Rakefile

The override for the fake forge to use only showed up in the
ci-bootstrap-from-artifacts.sh, but that only affects how the jenkins
jobs handle the pipeline. When you run the tests on your own machine,
you aren't going to use that script, which would mean that it would use
the wrong fake forge. This moves the override to the Rakefile so that it
will always take effect.
Merge pull request #2790 from zaphod42/issue/stable/pup-2816-using-wrong-fake-forge

(PUP-2816) Default forge override in Rakefile