[cloud] Check AWS instance type coverage on a weekly schedule

Allow for proactive detection of new instance types that are not
covered by the existing automated tests, by splitting the type
coverage check into a separate workflow that runs on a weekly basis.

A weekly schedule is arguably overkill given the frequency at which
new instance types are introduced, but the instance type check takes
only a few seconds to run and so the minimal cost is justified for the
sake of getting relatively rapid notification of new instance types.

Signed-off-by: Michael Brown <mcb30@ipxe.org>