Partially fix#6345, only PR portion, not nightly or releases.
Unlike travis, we're not caching our brew packages. `actions/cache` doesn't
update the cache on cache hit, making it hard to use a rolling cache like
before. We also don't know what cache key we should use before running, since
it relies on the live package list of brew.
Using a docker image containing brew packages seems like a better option if we
need the speedup going forward. ATM a full build with deps only takes about 12
minutes. Windows builds don't have this issue, since deps there are keyed off of
known versions in our repo.
Current Windows matrix form works, but causes a double-build on dep change due
to both debug/release rebuilding the release deps. Instead, should probably
separate the dep jobs, block build on the dep jobs, and update the cache on dep
jobs, guaranteeing a cache hit on build jobs?
Windows stage 1 and stage 2 can probably be recombined, if they were split due
to travis single build length limits, since GH actions allows much longer single
builds.
This commit takes the existing travis task list and attempts to split
it into multiple jobs for the sake of efficiency. Also properly
separates out the build/deploy of documentation.
