mambabuild errors when `downstreams` test results in an unsatisfiable environment

[ryanvolz]

When a test: downstreams: entry results in an unsatisfiable environment (commonly because the newly built package includes a version bump, and the downstream package would need to be rebuilt first), mambabuild should issue a warning and continue on, skipping the test, rather than failing and exiting. The logs are gone, but you can see https://github.com/conda-forge/gnuradio-feedstock/pull/97 for failed linux-64 builds due to a downstream test with mambabuild that then succeed after the build is switched back to conda-build only.