snap: use `snapctl model` to determine if Classic

[Saviq]

# on Core16..22
$ for i in 16 18 20 22; do multipass exec core$i -- snap run --shell ubuntu-frame -c "snapctl model" 2>&1 | awk '/^classic:/ { print $2 }'; done

# locally on Classic+TPM-FDE
$ sudo snap run --shell ubuntu-frame -c "snapctl model" 2>&1 | awk '/^classic:/ { print $2 }'                                                    
true