Add Jitter to "Update Go" action schedule

[phil9909]

Summary

Fixes #966 by adding a Jitter to the cron schedule (Alternative 2 in the issue)

The jitter is seeded with the repo name (GITHUB_REPOSITORY env) and will there produce a constant value for the same repo (unless renamed).

It is implemented in a separate package to be easily reused in other actions that might profit from this as well.

Use Cases

Prevent rate limiting

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[dmikusa]

Thank you very much for this. This was a high priority fix.

Two very minor notes around doc and I think this should be good to merge.

[dmikusa]

Looks great! Thanks very much!