fix: install the avahi .deb non-interactively past the conffile prompt

[ArnavVWadhwa]

Follow-up to #6. Now that the avahi_0.8 .deb installs over stock avahi (Replaces), the full image build hit the next wall: dpkg raised an interactive conffile prompt on /etc/avahi/avahi-daemon.conf and aborted.

Configuration file '/etc/avahi/avahi-daemon.conf'
 ==> Modified (by you or by a script) since installation.
*** avahi-daemon.conf (Y/I/N/O/D/Z) [default=N] ? dpkg: error processing package avahi-dpea (--configure):
 end of file on stdin at conffile prompt

The base RPi OS image ships an RPi-modified avahi-daemon.conf, and the .deb ships its own copy as a conffile, so dpkg wants a decision. DEBIAN_FRONTEND=noninteractive does not cover dpkg's own conffile prompt (that needs --force-conf*), so with no stdin the install died. #6's set -euo pipefail correctly turned this into a hard build failure instead of a silently broken image.

The verify-install gate in avahi_0.8 and this workflow both missed it because they installed over a pristine stock conf, which produces no prompt.

• customize.sh: install with -o Dpkg::Options::=--force-confdef -o Dpkg::Options::=--force-confold. Keep the base config (the 5358 patch is in the binary/lib, not the conf).
• validate.yml: append a line to /etc/avahi/avahi-daemon.conf before running customize.sh, so CI reproduces the modified-conffile prompt, not just the file-overwrite conflict.