Install ubuntu-keyring from apt again instead of using deprecated apt-key

[ryonakano]

Problem

We're using the deprecated apt-key during the build process:

https://github.com/elementary/os/blob/dcfbeb5229fbbce862c66e05c01ebb97a4faca14/build.sh#L33

You can see the warning if you build the image:

Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).
Executing: /tmp/apt-key-gpghome.sUZHX0hGxx/gpg.1.sh --recv-keys --keyserver keyserver.ubuntu.com F6ECB3762474EDA9D21B7022871920D1991BC93C
gpg: key 871920D1991BC93C: public key "Ubuntu Archive Automatic Signing Key (2018) <[email protected]>" imported
gpg: Total number processed: 1
gpg:               imported: 1

Proposal

Replace the deprecated apt-key command.

It looks like the ubuntu-keyring package came back to the debian repository since Trixie (see https://packages.debian.org/search?searchon=names&keywords=ubuntu-keyring), so maybe we can simply install it with apt when the debian:latest docker image is based on Trixie (it's based on Bookworm at the moment).

Prior Art (Optional)

#565 for the previous code that installs ubuntu-keyring from apt