No supported algorithms on platform

Job #3783240 failed for 5d20a7ce:

$ PYTHON="$(source bin/tests/system/conf.sh; echo $PYTHON)"
Traceback (most recent call last):
  File "/builds/isc-projects/bind9/bin/tests/system/get_algorithms.py", line 241, in <module>
    main()
  File "/builds/isc-projects/bind9/bin/tests/system/get_algorithms.py", line 227, in main
    algs = filter_supported(algs)
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/builds/isc-projects/bind9/bin/tests/system/get_algorithms.py", line 138, in filter_supported
    raise RuntimeError(
RuntimeError: no DEFAULT algorithm from "stable" set supported on this platform
$