setup-interfaces: use $iface with iw scan instead of wlan0

I have a Dell/Wyse 3040 thin client with factory wifi card which shows up as interface mlan0. Setup-interfaces fails when trying to scan for wireless networks because wlan0 is hard coded in the command:

https://gitlab.alpinelinux.org/alpine/alpine-conf/-/blob/master/setup-interfaces.in?ref_type=heads#L125

I changed it to use the already defined $iface variable and was able to successfully connect and finish the setup:

(iw dev "$iface" scan; echo BSS) | awk -F": " '