Filtered evals should also be treated as blocked in system scheduler

PR #5900 implemented support for blocked evaluations in the system scheduler, such that system jobs would be placed after capacity was available.

Some node properties are immutable (e.g. arch) and we do not want to recheck those properties for system jobs. Some are mutable (e.g. driver availability) and we need to recheck blocked evaluations when those properties change. We need to block filtered jobs iff they are filtered by properties that may change.

Start by reading PR #5900 and trace the system scheduler's blocked-evaluation handling and filtering logic. Identify how node properties are classified as immutable or mutable, then verify that filtered jobs are blocked only for mutable properties and rechecked when those properties change. Done means immutable-property filters are not rechecked while mutable-property filters are handled like blocked evaluations.

Written by the indexing model from the issue text.