CI: Cache the built node

[rt121212121]

Now that the node is built using a binaries directory that is decoupled from the source location, we should be able to directly point a Cache task at the BSV build sub-directory and packages should build almost immediately.

The big win is that this then makes rebuilding when there are only Python changes, almost immediate. And it makes making changes to tests, run promptly as well.

[rt121212121]

This would likely require:

• The cache directory being the extracted built objects that the package will include.
• The build process being changed to move the objects there.
• The setup.py script using this new cached object directory and skipping the build if it were present, rather than the cloned repo directory.
• The build scripts taking in an extra argument representing the revision to build (revision is a fixed hash which a tag is not).
• The cache key using the build revision and OS as part of it for uniqueness.

[rt121212121]

The plan was flawed. It can't possibly pull in the dependencies unless they are already present, and building Boost is part of the Linux build process this would skip.

• See issue Linux: Cache the boost build from within manylinux to the host and back #15 for Linux.
• Revisit all the build caching so that Windows and MacOS have the dependencies installed for auditwheel and delocate to pull in.