Fix inefficient Docker caching in CI

[gussmith23]

Seems like the Docker build isn't caching as nicely as it should. You may be able to view logs from this run:

https://github.com/uwsampl/lakeroad/actions/runs/8034507451/job/21946176589

This commit only changed things at/after the "build CVC5" stage, yet many of the steps are re-running. I can't currently see enough of the logs to know how far back it isn't caching, but at least from the Python dependencies section. The Python dependencies causing problems would make sense.