APT snapshots: add arm64 architecture

As discussed in feature 10972 port tails to arm, note 51 (see https://labs.riseup.net/code/issues/10972#note-51) please enable armhf and arm64 on APT snapshots.

Currently only armhf is required to test and adapt the live-build for arm, I have no arm64 platform.

Thanks!

Parent Task: #10972

Original created by @N9iu7pk on 12111 (Redmine)

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