cidata: skip set-name for slirp NIC to fix ubuntu-26.04 first-boot delay

On ubuntu-26.04 (and similar), systemd-networkd-wait-online is configured
by netplan to wait for the renamed interface (eth0) before it exists. On
first boot the NIC is still enp0s1 — the udev rename hasn't happened yet —
causing a 120s timeout.

Skipping set-name for the built-in slirp NIC leaves the interface under its
kernel-assigned predictable name, so systemd-networkd-wait-online no longer
waits for eth0 and boot completes immediately. set-name is preserved for
any user-configured extra NICs.

Fixes: #4792

Author: itsme-ranger

pkg/cidata/cidata.TEMPLATE.d/network-config

@@ -5,7 +5,9 @@ ethernets:
     match:
       macaddress: '{{$nw.MACAddress}}'
     dhcp4: true
+    {{- if ne $nw.Interface $.SlirpNICName }}
     set-name: {{$nw.Interface}}
+    {{- end }}
     dhcp4-overrides:
       route-metric: {{$nw.Metric}}
     dhcp-identifier: mac