Properly set python interpreter for Bullseye target hosts

Using the ansible.cfg interpreter_python does not work yet, see
https://github.com/mitogen-hq/mitogen/issues/740

.gitlab-ci.yml

@@ -53,7 +53,7 @@ base_test:
 base_bullseye_test:
   <<: *base_test
   variables:
-    CREATE_ENV_VARS: "-e config.float_debian_dist=bullseye -e ansible_cfg.defaults.interpreter_python=/usr/bin/python3"
+    CREATE_ENV_VARS: "-e config.float_debian_dist=bullseye -e inventory.group_vars.vagrant.ansible_python_interpreter=/usr/bin/python3"
     TEST_DIR: "test/base.ref"
 
 full_test: