Fix workflow cron syntax

[damccorm]

GitHub Actions doesn't support the same cron syntax as Jenkins - https://docs.github.com/en/actions/migrating-to-github-actions/migrating-from-jenkins-to-github-actions#scheduling-a-pipeline-to-run-with-cron

This fixes the problem leading to failed runs like https://github.com/apache/beam/actions/runs/3565663232

I also manually added my own jitters by generating random values for the day/hour fields (so this maintains parity with using H)

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[damccorm]

R: @AnandInguva @pabloem @alxmrs

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[AnandInguva]

Thanks!

[pabloem]

aw thanks <3