The UniFi GPG key at dl.ui.com/unifi/unifi-repo.gpg is already in binary
format. setup_deb822_repo unconditionally ran gpg --dearmor which expects
ASCII-armored input, corrupting binary keys and causing apt to fail with
'Unable to locate package unifi'.
setup_deb822_repo now downloads the key to a temp file first and uses
the file command to detect whether it is already a binary PGP/GPG key.
Binary keys are copied directly; armored keys are dearmored as before.
This also reverts unifi-install.sh back to using setup_deb822_repo for
consistency with all other install scripts.
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