No config set when using conda_build.api.build() directly from metadata

This MR implements a change in the use of conda_build.api.build() in the specific cases we are building from the resolved metadata. It reverts using metadata to run the build as that has proved not be reliable.

See:

• https://gitlab.idiap.ch/bob/bob.measure/-/jobs/159689
• closes bob/bob.conda#68