sylva-core branch detection wrongly select a dev branch

reported by Frédéric Leroy:

I can no longer bootstrap v1.6.8. I have this error on kind:

sylva-units-preview           5m50s       Warning    GitOperationFailed        gitrepository/sylva-units-6a910712
        failed to checkout and determine revision:
unable to clone 'https://gitlab.com/sylva-projects/sylva-core.git':
couldn't find remote ref "refs/heads/ak-heal-dep-1-6" on 
Validate sylva-units values for management cluster