Have PTB use `workflows:update` to update its workflows

[ArBridgeman]

Summary

Modify the PTB so that it uses workflows directly from workflows:update. This should already be the same except for:

• exasol/python-toolbox/.github/actions/python-environment@vx is ./.github/actions/python-environment
• checks.yml where fetch-depth:0 is needed for the unit tests

Historically, this was not the case, & instead, developers copied out the needed changes. It's important that we do this as workflows:check was added to the workflow template in #639 & so that we can test the templates execute on GitHub as expected.

To Dos

• Create a custom workflow to separately test that ./.github/actions/python-environment works. Make sure this is required in the CI build chain. In the workflows that are templates, please then use exasol/python-toolbox/.github/actions/python-environment@vx/.
• Handle other issues that arise from Mark workflow templates as generated or modified #639. It's likely that checks.yml and slow-checks.yml were marked as modified so that the CI check would not fail.