Find some way to cache build dependencies in Github Action

[OrdoFlammae]

Most of the time and processing in the automated action is spent in re-downloading the same dependencies every time there is an update. This is absurdly over the top for what is needed for an automatic build, and it would be nice to be able to skip this step in many/all cases. I am currently leaning towards a Docker-based solution, but it is possible that Github Actions' native cache system will work better. More investigation into exactly which system to use is needed.