Update pull_request.yml with new goodies from Codecov

[petrsvihlik]

Overwriting pull_request_target finally works in Codecov:

• https://github.com/codecov/codecov-action/releases/tag/v1.3.2
• codecov/codecov-action#264

The pull request workflow can be simplified now.

[petrsvihlik]

[petrsvihlik]

masking https://docs.github.com/en/actions/reference/workflow-commands-for-github-actions#masking-a-value-in-log

[petrsvihlik]

fixed in 842e6ff