Improvise on build script and build options.

[roshkhatri]

Currently there is no option to run the build.sh script for release and mentions threads at the build script level to specify the threads used for builds.

We would want to add option to build release only, where

1. we only build the module and not build valkey server.
2. not build the tests for release options.

[natoscott]

I think the title here should be "Improve" not "Improvise"?

What's the reason for downloading and building valkey itself as part of the valkey-json build? Can this be removed entirely instead of just for release builds, as described in point 1? The valkeymodule.h was supposed to allow the module to be used with any version of valkey, so I don't understand the need to download and build valkey ... worse, it adds complexity and massively slows the build down of course.