CI: use a fixed version of chutney

CI: use a fixed version of chutney

While chutney currently runs tor's chutney test in its own CI,
it's difficult to guarantee the two won't accidentally diverge.
Probably best to use a fixed version here so that we can control
chutney version bumps and avoid surprise breakage in tor's CI.

This will also free us to intentionally make breaking changes in
chutney (though I don't have any immediate plans for any).