Maintenance: Move per-test run coverage control to .github shared workflows

[mwiebe]

Description

In #758, I worked out a way to use a lower coverage target for Python 3.8, because a feature needed Python >= 3.9 so its tests do not run there. This involved copying the file https://github.com/aws-deadline/.github/blob/mainline/.github/workflows/reusable_python_build.yml here to https://github.com/aws-deadline/deadline-cloud/blob/mainline/.github/workflows/reusable_python_build_coverage_override.yml and adding a parameter to override the coverage option.

The result of this is that the github workflows are more complicated here, and this capability isn't available across all our repos.

Solution

Move the ability to override the coverage fail-under back to the aws-deadline/.github repo, by making the cov-fail-under parameter optional and using conditionals in the Run Tests step in the file https://github.com/aws-deadline/.github/blob/mainline/.github/workflows/reusable_python_build.yml. Then remove the copied workflow from deadline-cloud, and use the shared one instead.