Make CI using downstream integrations optional

[baentsch]

CI of this project also uses downstream packaging of liboqs (apt and brew) to test the integrity of the code base. This fails when a feature not yet packaged up downstream (ML-* in this case) is used.

This issue therefore proposes to make such tests optionally selectable, e.g. by environment variable, such as to permit CI to be adapted to the progress outside of the control of this project.

As usual, other proposals to resolve welcome.