keep-workflows-enabled: query for scheduled GH Action workflows

[joverlee521]

Context

Original feedback from @tsibley in #118 (comment)

I'd much prefer to query these (e.g. from GitHub's API) rather than hardcode them all here... (but we don't need to do that now if you aren't so inclined)

Description

Replace the hard-coded list of workflows with API query to find all Nextstrain GH Action workflows that have a schedule event to automatically keep them enabled. This will lessen the burden of need to update the central list whenever we add a new scheduled workflow.