terraform: disable cloudfront root object for crates.io on staging

There is a concern that setting the default root object to index.html
for the crates.io webapp causes Fastboot to break:

    rust-lang/crates.io#1937 (comment)

This adds a temporary feature flag to remove the default root object
just from staging, without affecting production. After testing is done
the feature flag will have to be removed, either disabling or enabling
the root object both on staging and production.

terraform/services.tf

@@ -103,6 +103,7 @@ module "service_cratesio" {
   iam_prefix = "crates-io"
 
   logs_bucket = aws_s3_bucket.temp_logs_cratesio.bucket_domain_name
+  temp_flag_root_object = true
 }
 
 module "service_cratesio_staging" {
@@ -122,6 +123,8 @@ module "service_cratesio_staging" {
   webapp_origin_domain = "staging-crates-io.herokuapp.com"
 
   iam_prefix = "staging-crates-io"
+
+  temp_flag_root_object = false
 }
 
 module "service_docsrs" {

terraform/services/cratesio/cloudfront-webapp.tf

@@ -6,7 +6,7 @@ resource "aws_cloudfront_distribution" "webapp" {
   enabled             = true
   wait_for_deployment = false
   is_ipv6_enabled     = true
-  default_root_object = "index.html"
+  default_root_object = var.temp_flag_root_object ? "index.html" : null
   price_class         = "PriceClass_All"
 
   aliases = [var.webapp_domain_name]

terraform/services/cratesio/variables.tf

@@ -36,3 +36,7 @@ variable "logs_bucket" {
   type        = string
   default     = null
 }
+
+variable "temp_flag_root_object" {
+  type = bool
+}