.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.

Fixes #859.

.gitlab-ci.yml

@@ -61,7 +61,7 @@ tests-ubuntu-18.04:
   <<: *tests
 
 overnight-fedora-28-aarch64:
-  image: buildstream/testsuite-fedora:aarch64-28-06bab030-32a101f6
+  image: buildstream/testsuite-fedora:aarch64-28-5da27168-32c47d1c
   tags:
     - aarch64
   <<: *tests