I once interviewed a candidate who decided to use Pharo to solve my technical problem. It was an absolute joy to see not only a good approach, but such an interesting and alien language employed to solve it.
We give onsite candidates affordances to install any environment that makes them feel comfortable, or to allow them to bring their own machines. The interview machines get wiped on a regular cadence, so installing packages is no big deal.
If candidates send us a list of the software they want beforehand, we usually try to set it up for them in advance. If not, it's fine for the schedule to run over a bit for them to tweak things as they like.
Interviews are stressful enough. We want everything working in a candidate's favor so they can do their best.
He got the job. :)