upgrades: Allow FreedomBox vendor when adding backports
Fixes: #2294 (closed).
Tests:
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In stable container, when frequent feature updates option is enabled, /etc/apt/sources.list.d/freedombox2.list exists as expected.
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Matrix Synapse can be installed.
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Shaarli can be installed.
Signed-off-by: James Valleroy jvalleroy@mailbox.org