I think I know why this is failing.

It's because of the virt unit tests.
I marked the make check step as optional for the github action . For debian sid (i.e. unstable), the --disable-virt configure flag is set (in the GHA experimental workflow), so it doesn't try to test virt (sid fails for other reasons though, but we don't expect that it'll always pass).
For bullseye, the virt plugin is not disabled, and make check actually fails (but is hidden because that step is marked as continue-on-error: true).

In order to use the bullseye_amd64 container here, you need to pass the --disable-virt flag to the ./configure step in cirrus.

(using buster should work, since the issue that causes virt unit tests to fail does not seem to be present there)

EDIT:
For GitHub Actions/build workflow:
If the --disable-virt flag is passed as a config flag in the task for bullseye and sid, we can remove the continue-on-error: true option for the make check step in the PR/merge jobs.