Come up with a better strategy for building bitfiles

[Aaron-Hartwig]

Right now our CI is basically "can we build every bitfile target cobble sees". This means that our minimum lap time is ~45 minutes while we build everything. I think there are a couple of layers to attack here.

1. Don't build every single bitfile for PRs and main. We should have a list of targets we actually want built (ignition targets, Sidecar controller, Gimlet Sequencer, QSFP controller). Others that are non-product focused should not get built every time. This would let us keep their targets around.
2. Leverage the build system to not rebuild things that were not impacted by a change.