Let post-pve-install go on when legacy sources are kept (#18403)

On a PVE 9 host without deb822 .sources files the script asks whether to
disable the legacy .list sources, then read the answer from $? on the
next line. Under set -e a "No" ended the whole run with exit 1 right
there, so the "Kept legacy sources as-is" branch never ran and nothing
after it was offered. Test whiptail in the if instead.

tools/pve/post-pve-install.sh

@@ -226,9 +226,8 @@ start_routines_9() {
         [[ -n "$list_files" ]] && MSG+="$(echo "$list_files" | sed 's|^| - |')\n"
         MSG+="\nDo you want to disable (comment out/rename) all legacy sources and use ONLY deb822 .sources format?\n\nRecommended for Proxmox VE 9."
 
-        whiptail --backtitle "Proxmox VE Helper Scripts" --title "Disable legacy sources?" \
-          --yesno "$MSG" 18 80
-        if [[ $? -eq 0 ]]; then
+        if whiptail --backtitle "Proxmox VE Helper Scripts" --title "Disable legacy sources?" \
+          --yesno "$MSG" 18 80; then
           # Backup and disable sources.list
           if [[ -f "$listfile" ]] && grep -qE '^\s*deb ' "$listfile"; then
             cp "$listfile" "$listfile.bak"