"Build the build requires" step should be optional

[jgsogo]

That step is not needed if all build-requires are available for the default configuration: if we are using a stable docker image from ConanCenter.

Approach with --build missing doesn't always work for transitive build-requires (see conan-io/conan#10345)

It should be possible to inspect (jq) and check if any build-requires are missing. If some are missing, then trigger the step, if not skip it.