[tex-live] How to add a key to the tlmgr gpg keyring persistently?

[Markus Kohm]

I've added a public key for an additional repository to the keyring used by 
tlmgr. After `tlmgr update -self' the keyring has been "updated" too and the 
manually added key has been "removed". After this I've got an fatal error, 
because the signature of the additional repository cannot be checked.

So how can I add a key to the keyring without need to add it again and again 
after each update of the TeX Live infrastructure files?
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