fix: tolerate nmcli connection.filename failure for in-memory live connections

export_connection_profile() crashed the whole installer under set -e/
pipefail when the active NetworkManager connection has no backing
keyfile (common for ephemeral live-boot DHCP connections) - nmcli
returned a non-zero exit status even though the guard below already
handles an empty result gracefully. Every other nmcli call in this
function already has this || true safety net; this one was missing it.

Found via a live end-to-end boot test in Proxmox.

scripts/modules/05_network.sh

@@ -440,7 +440,7 @@ export_connection_profile() {
         "${NMCLI}" \
             --get-values connection.filename \
             connection show "${connection_name}" 2>/dev/null |
-            head -n 1
+            head -n 1 || true
     )"
 
     if [[ -z "${source_file}" || ! -f "${source_file}" ]]; then