NIFI-9233 - Improve reliability of system integration tests

[greyp9]

Description of PR

Alter CI logic of system-tests such that they are only run on a schedule on the base repository. This is to avoid resource contention caused by repo forks also scheduling system tests to run at the same time.

When we re-enable system tests to run on pushes, we might re-evaluate this approach. It should be possible to run system tests on forks, just not on a (shared) schedule. The situation we want to avoid is having all enabled forks (potentially hundreds) simultaneously requesting macos and linux build nodes for an operation that is expected to take ~1 hour. Hopefully this will help the system-test suite to run with greater stability.

In order to avoid the potential for resource contention at 0000Z with existing forks (who have not pulled this change), adjust the daily schedule trigger time.

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Note:

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[greyp9]

We eventually want to re-enable the system tests for pull requests. There doesn't seem to be a way to limit the cron trigger to a certain owner, while allowing all repo owners on a different trigger. Cancelling this PR.

https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onschedule