Only CC arches when blocker bugs have CC-ARCHES too

[thesamesam]

Suppose we have bugs A and B:

• A blocks B, i.e. stabilisation bug B needs something stabilised in bug A
• bug A has a valid sanity check and package list, but no CC-ARCHES yet (e.g. waiting on maintainer ack)
• bug B has a valid sanity check and package list and has CC-ARCHES

Right now, nattka will CC arches in bug B, even though it can't actually be done yet.

Example:

• bug A is https://bugs.gentoo.org/913962 - Rust 1.71.1 stabilisation, no arches CCed yet but green sanity check
• bug B is https://bugs.gentoo.org/913961 - eza stabilisation which needs >=rust-1.70.0 which isn't yet stable, but depends on bug 913962, has CC arches, nattka CCed amd64 despite it not being actionable yet