Since the pipeline is meant to be `include:`d from other repositories
by default it tries to retrieve the `update_test_binaries.sh` script
with a HTTP call.

However this means that the test pipeline on a specific commit is not
actually testing the `update_test_binaries.sh` script on the same
commit, but it retrieves it from the `$osname/$release` branch (in this
case, `apertis/v2023dev1`).

Tweak things to ensure the current `update_test_binaries.sh` is used
during testing.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>