Update CPU threads dedicated for build

[kylelondon92]

Changed the static threads count assigned in "ci/actions/build.sh" to dynamically pick up all the threads available for faster builds.

[zhyatt]

Thanks @kylelondon92 . This implementation is currently not used as it was intended for use with the Nano PoW algorithm which was put on hold last year. The infrastructure may still be used for a future PoW algorithm change, but in the meantime you'll want to use the internal node work generation or https://github.com/nanocurrency/nano-work-server for your work generation needs. There is also a helpful guide in the docs worth reviewing. I will leave this open for now.