non-deterministic test failure

This should probably be investigated. It seems like we're hitting a recursion sometimes https://gitlab.com/BuildStream/buildstream/-/jobs/343043927

=================================== FAILURES ===================================
____________________________ test_cyclic_variables _____________________________
[gw0] linux -- Python 3.7.5 /builds/BuildStream/buildstream/.tox/py37/bin/python

>   expstr = [sys.intern(_expand_expstr(self._expstr_map, expstr))]

src/buildstream/_variables.pyx:187: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

>   _expand_expstr_helper(content, topvalue, result)

src/buildstream/_variables.pyx:282: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

>   _expand_expstr_helper(content, <list> content[value[idx]], acc, counter + 1)

src/buildstream/_variables.pyx:262: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

>   _expand_expstr_helper(content, <list> content[value[idx]], acc, counter + 1)
E   RecursionError

src/buildstream/_variables.pyx:262: RecursionError
!!! Recursion detected (same locals & position)