.gitlab-ci.yml: Use newer image for overning aarch tests

During recent reorganizing of `.gitlab-ci.yml` to work with `tox`, seems
like we missed to update the image used by the overnight aarch tests,
meaning that they currently fail due to `tox` being missing from them.

While these tests will be skipped on MRs usually, here is an example of
what the tests look like if they are actually run with this change:
https://gitlab.com/BuildStream/buildstream/-/jobs/145449561.

Fixes #859.