[boost] Github Actions slow

On https://github.com/boostorg/json we have noticed very slow Github Actions times such as "Total duration: 6h 8m 47s" This is very likely caused by Github throttling jobs. https://docs.github.com/en/actions/learn-github-actions/usage-limits-billing... mentions limits of between 20 and 180 which would apply to the entire boostorg account. A reminder to any boost developers who are seeing slow processing time that they may use Drone CI, instructions at https://github.com/boostorg/boost-ci