fix(ci): keep appcast history when a previous release has no assets

[davidpoblador]

Problem

The appcast step merges items from a previous release so the Homebrew update
popover keeps its history. It picked the most recent release other than the one
being built:

PREV_TAG=$(gh release list --limit 10 --json tagName --jq "[.[] | select(.tagName != \"$TAG_NAME\")][0].tagName")

If that release has no assets, the download fails, || true swallows it, and
generate_appcast.py guards on os.path.exists, so the build happily publishes
an appcast containing only its own item.

That is not hypothetical: it happened on 0.1.78. Its build ran after 0.1.77,
whose build had failed and left the release empty, so the published 0.1.78
appcast has exactly 1 item where it should have had 77.

Sparkle updates still work, since only the newest item matters for updating.
What breaks is the version history.

Fix

Walk back through recent releases until one actually yields an appcast, and emit
a warning instead of truncating silently if none does.

Verification

Ran the new loop against this repository with TAG_NAME=v0.1.78:

Carrying forward appcast items from v0.1.76
items carried forward: 76

It skips the empty v0.1.77 and recovers the history. The old code stopped at
v0.1.77 and carried nothing.

Note this does not retroactively repair the 0.1.78 appcast. The next release
picks up the history again, because v0.1.78 does have an appcast.xml asset, so
it self-heals from here.