[boost] Github releases for CI

In addition to the CMake releases on Github, I've modified the action to also create releases for use with b2 (with the headers copied to boost/ and removed from libs/*/include, but without the documentation.) These would be useful for CI, which would hopefully lower the pressure on the official downloads. E.g. https://github.com/boostorg/boost/releases/tag/boost-1.85.0.beta1