Motivation:

* Improve test coverage
* Check test pass/fail status
* Monitor CI throughput

Modifications:

Enable macOS CI to be run on all merges to main and on a daily timer.

Result:

Improved test coverage run out-of-band at the moment so we can get a
feeling for if any changes need to be made in the repo or in the CI
pipelines to ensure timely and stable checks.

Update main.yml indentation

Only run Darwin CI on macOS targets as that's the only supported one.

Motivation:

* Improve test coverage

Modifications:

Enable macOS CI to be run on pull request commits and make the use of
the nightly runner pool for main.yml jobs explicit.

Result:

Improved test coverage.