There clingo-cffi job has two issues to be solved:
1. It uses the default concretizer
2. It requires a package from https://test.pypi.org/simple/
The former can be fixed by setting the SPACK_TEST_SOLVER
environment variable to "clingo".
The latter though requires clingo-cffi to be pushed to a
more stable package index (since https://test.pypi.org/simple/
is meant as a scratch version of PyPI that can be wiped at
any time).
For the time being run the tests in a container. Switch back to
PyPI whenever a new official version of clingo will be released.
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