Add external IP to instance template

Previously, instances, created by the instance template, were unable to
download the launcher binary, because they had no external IP addresses
assigned.

This PR sets the network tier to `PREMIUM`, which assigns an ephemeral
IP address to the created instances automatically.

main.tf

@@ -92,6 +92,9 @@ resource "google_compute_instance_template" "spacelift-worker" {
   network_interface {
     network    = var.network
     subnetwork = var.subnetwork
+    access_config {
+      network_tier = "PREMIUM"
+    }
   }
 
   service_account {
@@ -112,7 +115,7 @@ resource "google_compute_instance_template" "spacelift-worker" {
 }
 
 resource "google_compute_instance_group_manager" "spacelift-worker" {
-  name    = var.instance_group_manager_name
+  name = var.instance_group_manager_name
 
   base_instance_name = var.instance_group_base_instance_name
   zone               = var.zone