Dispatch job not compatible with main default branch

Summary

The job was configured to consider master as the default branch.

However, the default branch is changed to main since a while in GitLab: https://about.gitlab.com/blog/2021/03/10/new-git-default-branch-name/

We have to adapt the configuration to work with both branch names thanks to the CI_DEFAULT_BRANCH env variable.

Ref: https://gitlab.com/nexylan/skel/blob/c98e9351e1c672a02f81886dd9ec0a7c65b2f872/rootfs/bin/dispatch#L8