Limit scheduled jobs in scan-for-updates.yml and test-all-and-report.yml to gap-system/PackageDistro

[fingolfin]

It is annoying that these workflows run hourly/daily in forks of this repository, too.

We could insert a check that aborts these workflows if they are run in a fork AND if the trigger is a schedule: We still want to allow manual triggers so that one can test these workflows in forks. (And if one wants to test a schedule, one can temporarily disable the logic that skips the workflow in forks)