ci: re-fetch moving-ref (VERSION=HEAD) package downloads each build

[widgetii]

Problem

Packages pinned to a moving ref (VERSION = HEAD / a branch) download into the
buildroot dl cache under a constant filename — <pkg>-<ref>.tar.gz (e.g.
majestic-webui-HEAD.tar.gz). The dl cache is keyed on the month
(CACHE_DATE=$(date +%m)) and actions/cache only saves on a key miss, so the
first nightly of each month freezes output/dl for weeks. From then on buildroot
finds the constant filename already present and never re-fetches — the nightly
silently ships a weeks-old snapshot of every moving-ref package, while
content-addressed packages (majestic from S3, semver/SHA pins) keep refreshing.

This desynced a stale-cached majestic-webui from a fresh majestic: the
streamer's config schema moved field labels to a new key, but the frozen webui
still read the old one, so every field/switch label in the settings UI went
blank on affected nightlies. ~28 GitHub packages (ipctool, divinus, go2rtc,
msposd, the wifi/SDK drivers, …) share this same latent staleness hazard.

Fix

Before the build, delete the cached moving-ref archives so buildroot re-fetches
the current ref each run:

find output/dl -type f \
  \( -name '*-HEAD.tar.gz' -o -name '*-master.tar.gz' -o -name '*-main.tar.gz' \) \
  -print -delete

Content-addressed downloads (S3 / semver / SHA pins) keep their cache untouched,
so this only re-fetches the handful of packages that are supposed to track a
moving ref. Runs right after the dl-cache restore, before Build firmware.

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