The new yarn's repository signing key was not recognized via the
old way to declare the key.
Using the new recommanded way[1] solves the problem.
It's the opportunity to migrate the declaration of the other
repositories to this method.
[1] https://wiki.debian.org/DebianRepository/UseThirdParty
Related to T3120
If we are ok with this way, perhaps we should also change the way the key of
the other repositories are managed