Reduce CI testing time by limiting tests for draft PRs

Try to configure CI to run macOS/Windows tests only on ready-for-review PRs to increase turnover and reduce pressure on concurrent jobs when still in draft phase. This can (maybe) be achieved by looking at the `github.event.pull_request.draft == true/false` property.

Should only be considered once #809 is merged.

Xref: https://github.com/trixi-framework/Trixi.jl/pull/809#issuecomment-904788837

Start with the repository's CI workflow configuration and inspect how pull-request events are handled, using the mentioned github.event.pull_request.draft property as the entry point. After #809 is merged, confirm that macOS and Windows tests are skipped for draft PRs and run for ready-for-review PRs.

Written by the indexing model from the issue text.