feat(ci): avoid creating jobs for outputs that are cached (#1218)

- Along side unionlabs/workflows#27, this PR makes it so outputs that
are already cached won't have jobs spawned for them
- Adds `mkCi` which can be used to disable CI for an output from its
deceleration in nix.
- Outputs that should be enabled/disabled per system can also now be
filtered with this derivation.