[ci] Better bitstream build cache filtering

[nbdd0121]

We want to avoid rebuilding bitstreams if possible. To do this we look at the list of changed files and determine if a rebuild is warranted.

Currently the filtering is entirely hardcoded rules. This PR uses Bazel to get a list of dependencies required for the bitstream, and filter the changes based on that list.

[rswarbrick]

Hi @nbdd0121! Is this still something that we should be doing? It looks sensible to me: maybe it's worth rebasing?

[nbdd0121]

I think I still want to pursue this, but it's unlikely that this could be merged before the quiet period, so I'll defer this to a later date.

[nbdd0121]

Apparently now building the bitstream does not require opentitanlib, so we can now ask Bazel to find dependencies and do extra filter on top without too much faff.