ci: switch to 'ubuntu-20.04-self-hosted' run label

The idea of these changes is to run testing workflows on self-hosted
ubuntu-20.04 machines instead of GitHub ubuntu-20.04 runners. It is
planned to use GitHub ubuntu-20.04 runners only for integration testing
of tarantool with a module/connector where ephemeral environments will
allow us avoiding various cleanup pains on self-hosted runners like

    - if: always()
      run: sudo apt-get -y purge 'tarantool*'

or

    - if: always()
      run: rm -rf .rocks

or

    - if: always()
      run: rm -rf pytest-venv

Also, switch workflows running on GitHub ubuntu-18.04 runners to
self-hosted ubuntu-20.04 machines.