fix(guide): full-sync arch in the boot image instead of a partial -Sy

The guide job builds its boot image from a pinned pkgmgr-arch parent that
can be weeks old, and `pacman -Sy systemd` against it is the partial
upgrade arch upstream documents as unsupported. It pulled systemd 261.2-1
next to the image's systemd-sysvcompat, which pins systemd exactly, so the
transaction could not be satisfied and every arch guide run died before
the README sequence started.

Refreshing archlinux-keyring first and then running a full -Syu --needed
fixes it; the cache wipe matches what the apt and dnf branches already do.
The upgrade lands in the PREP container, which runs sleep infinity, so
systemd is only ever replaced before it becomes PID 1 in the committed
image.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/github/guide/host_deploy.sh

@@ -28,7 +28,9 @@ docker exec "${PREP}" sh -c '
 	elif command -v dnf >/dev/null 2>&1; then
 		dnf install -y systemd
 	elif command -v pacman >/dev/null 2>&1; then
-		pacman -Sy --noconfirm systemd
+		pacman -Sy --noconfirm --needed archlinux-keyring
+		pacman -Syu --noconfirm --needed systemd
+		rm -rf /var/cache/pacman/pkg/*
 	fi
 	grep -q myhostname /etc/nsswitch.conf || sed -i "s/^hosts:.*/& myhostname/" /etc/nsswitch.conf
 	[ -e /sbin/init ] || ln -sf /lib/systemd/systemd /sbin/init