cmd-init: use sudo when creating src/config.json

When following the steps for building SCOS I ran
`cosa init --variant scos https://github.com/openshift/os.git`
and got a failure. Let's run this with `sudo`.

src/cmd-init

@@ -192,7 +192,7 @@ if [[ -n "${VARIANT}" ]] && [[ "${VARIANT}" != "default" ]]; then
         fatal "Could not find the manifests (${manifest} & ${image}) for the '${VARIANT}' variant"
     fi
     echo "Using variant: '${VARIANT}'"
-    cat > "src/config.json" <<EOF
+    sudo tee "src/config.json" <<EOF
 {
   "coreos-assembler.config-variant": "${VARIANT}"
 }