Update Python environment setup in GitHub Actions (#283)

* CI updates

* try fix

* use matrix strategy

* fix

* fix again

* fix

* try fix

* try fix

* fix

* fix

* fix again

* start venv with .

* more fixes

* try again

* run config checks on self-hosted

* update docs and build jobs too

* try env hack

* try again

* reset cache

* fix

* Trigger CI again

* updates

* clean up

* clean up

* set cache prefix at global env level

* use restory keys

* clear cache weekly

* switch Docker job to 3.8

* add clean up step