CI: Don't use a venv for either GitHub Actions or CircleCI

[makyen]

Upon additional testing, it was seen that there needed to be more done in each CI run in order to make sure that a cache of the venv stayed at the most recent versions of our sub-dependencies without exceeding the versions required for our explicit requirements. The extra steps are required because pip doesn't update sub-dependencies of explicitly specified dependencies, unless required. Doing the extra steps cost enough time such that using a venv is, at best, neutral with respect to the amount of time required for installing all of the requirements from cached copies of the package downloads. In multiple cases, using the cached venv actually took longer. The entire point of using a venv in our CI was that fully caching the installed dependencies resulted in a savings in the run time for the CI tests. Without there being a time savings, there isn't a point to using a venv.

So, this PR removes the use of venv from both GitHub Actions and CircleCI. It implements/re-implements caching for the main pip download cache.

Resolves #7100