Update instructions to use a fixed SHA for Tinker Engine checkout
This MR makes the installation procedure for the Learn framework more reproducible by prescribing a specific SHA checkout for Tinker Engine.
In the future, this can be updated to make use of release tags instead of a SHA. That can be done after Tinker Engine has a good release procedure in place.
Wait to merge this until after https://gitlab.kitware.com/darpa_learn/framework/-/merge_requests/35 is merged.