Rework build scripts

[mateuszdrwal]

Currently, one can specify a build script that is ran when the challenge is built with a flag as a command line argument, allowing for the flag to only be defined in challenge.yaml. The intention has always been for build files to be committed to the challenge repository (so the builder doesn't have to replicate the same environment the problem author has to push challenges), and for the feature to only be used for inserting flags. I have though seen instances where people use this to just run make.

I'm not sure what the best way is to discourage actual builds in the build scripts. The name is very confusing for its purpose, so it should probably be renamed if it stays like this. Alternatively this could be replaced by a flag file option which just writes the flag to a specified path on build, but this prevents legitimate use cases of build scripts where the flag is built into the binary. Perhaps some sort of build docker container is in order, or the ability to copy out files from the image build?