Force IPv4 in the build VM and fix the APT cache path

The build logs here show the same stall measured on maas-samba-ad: a 28.5 MB
apt fetch taking 31s and a 12.0 MB one taking 30s, while a 14.1 MB fetch
completed in under a second. A fixed cost that ignores size is a timeout, not
a bandwidth limit — QEMU's user-mode network advertises IPv6 that does not
work, so apt's parallel connections black-hole on it and fall back to IPv4
only when the 30-second timeout expires.

Patch the build VM's cloud-init seed from bootcmd, which runs before SSH is
up and therefore covers upstream's apt calls too. On maas-samba-ad this took
the same 28.5 MB fetch from 31s to 3s; no build has been run here since, and
the README says so.

APT_PROXY also never worked as documented: a cache cannot see inside a CONNECT
tunnel, so repositories must be rewritten to plain http, and Debian 13 keeps
the real mirror URLs in /etc/apt/mirrors/*.list behind the mirror+file:
method, which the old sed missed.

Drop the invented "roughly 700 MB" saving from the README. On maas-samba-ad a
fully warm cache was worth about three seconds of a 4m40s build. This image
pulls far more from the Proxmox repository, so the cache may matter more here,
but that is unmeasured and is now listed as such.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/customize-proxmox.sh.in

@@ -74,8 +74,11 @@ apt-mark hold cloud-init || true
 # Paket imzalari yine dogrulandigi icin guvenlik kaybi yok.
 if [ -n "${http_proxy:-}" ]; then
     log "APT onbellegi kullaniliyor (${http_proxy}); Debian depolari http'ye cevriliyor"
-    sed -i 's|https://deb.debian.org|http://deb.debian.org|g; s|https://security.debian.org|http://security.debian.org|g' \
-        /etc/apt/sources.list /etc/apt/sources.list.d/*.sources /etc/apt/sources.list.d/*.list 2>/dev/null || true
+    # Debian 13 keeps the actual mirror URLs in /etc/apt/mirrors/*.list, reached
+    # through the "mirror+file:" method, so rewriting sources.list alone misses them.
+    sed -i 's|https://|http://|g' \
+        /etc/apt/sources.list /etc/apt/sources.list.d/*.sources \
+        /etc/apt/sources.list.d/*.list /etc/apt/mirrors/*.list 2>/dev/null || true
 fi
 
 log "eatmydata kuruluyor"