Avoid deprecated "apt-key add", instead drop keys in /etc/apt/trusted.gpg.d

This is a backport of the relevant part of these 2 upstream commits:

• https://salsa.debian.org/live-team/live-build/-/commit/e5492b1c702858eb26e2b93c65810773ad0bfa85#eb0d994b6e811f41415f6ae45d3336eca4e3ddee
• https://salsa.debian.org/live-team/live-build/-/commit/8403487d4e3bda65cdd2ea6081399f7977325adb

Original authors: Marcel Partap, Luca Boccassi

Closes tails#19268 (closed)