Custom Linux VM images broken by changes to apt_sku in locals.tf

Describe the bug
locals.tf calculates apt_sku with a dynamic lookup from the selected image. The configuration for custom images provides no mechanism for passing a value for this. Attempting to boot custom images therefore fails before terraform manages to do anything.

Steps to reproduce

1. Define custom images in your TRE
2. Attempt to boot a custom Linux VM
3. Wait for the failure message referring to apt_sku

In the past, there was a line checking to see if apt_sku was defined on the selected image, and substituting unknown_sku if it wasn't. Restoring this line resolves the terraform problem, and the machine will boot:

vscode ➜ .../guacamole/user_resources/guacamole-azure-linuxvm/terraform (main) $ git diff locals.tf
diff --git a/templates/workspace_services/guacamole/user_resources/guacamole-azure-linuxvm/terraform/locals.tf b/templates/workspace_services/guacamole/user_resources/guacamole-azure-linuxvm/terraform/locals.tf
index a7f326ef..72c98bfb 100644
--- a/templates/workspace_services/guacamole/user_resources/guacamole-azure-linuxvm/terraform/locals.tf
+++ b/templates/workspace_services/guacamole/user_resources/guacamole-azure-linuxvm/terraform/locals.tf
@@ -25,7 +25,8 @@ locals {
   # selected_image_source_refs is an array to enable easy use of a dynamic block
   selected_image_source_refs = lookup(local.selected_image, "source_image_reference", null) == null ? [] : [local.selected_image.source_image_reference]
   selected_image_source_id   = lookup(local.selected_image, "source_image_name", null) == null ? null : "${var.image_gallery_id}/images/${local.selected_image.source_image_name}"
-  apt_sku                    = local.selected_image_source_refs[0]["apt_sku"]
+  # apt_sku                    = local.selected_image_source_refs[0]["apt_sku"]
+  apt_sku = length(local.selected_image_source_refs) > 0 ? local.selected_image_source_refs[0]["apt_sku"] : "unknown_sku"
 
   cmk_name                 = "tre-encryption-${local.workspace_resource_name_suffix}"
   encryption_identity_name = "id-encryption-${var.tre_id}-${local.short_workspace_id}"

However, that leads to another problem. The value of apt_sku is used to configure Nexus to point to the correct Ubuntu repository, being instantiated by using apt-sources_config.yml as a template.

A robust way to fix this is to use the VERSION_ID from /etc/os-release at boot-time to correct the unknown_sku:

vscode ➜ .../guacamole/user_resources/guacamole-azure-linuxvm/terraform (main) $ git diff vm_config.sh
diff --git a/templates/workspace_services/guacamole/user_resources/guacamole-azure-linuxvm/terraform/vm_config.sh b/templates/workspace_services/guacamole/user_resources/guacamole-azure-linuxvm/terraform/vm_config.sh
index 1c1f918f..4a2da16e 100644
--- a/templates/workspace_services/guacamole/user_resources/guacamole-azure-linuxvm/terraform/vm_config.sh
+++ b/templates/workspace_services/guacamole/user_resources/guacamole-azure-linuxvm/terraform/vm_config.sh
@@ -9,6 +9,10 @@ set -o xtrace
 # Remove apt sources not included in sources.list file
 sudo rm -f /etc/apt/sources.list.d/*
 
+# Fix Ubuntu version in APT sources for custom images
+. /etc/os-release
+sed -i "s%unknown_sku%$VERSION_ID%" /etc/apt/sources.list
+
 # Update apt packages from configured Nexus sources
 echo "init_vm.sh: START"
 sudo apt update || true

Azure TRE release version (e.g. v0.14.0 or main):

Head as of middle of last week.

Deployed Azure TRE components - click the (i) in the UI:
UI Version: 0.6.3
API Version: 0.20.3

Start with templates/workspace_services/guacamole/user_resources/guacamole-azure-linuxvm/terraform/locals.tf and vm_config.sh. Trace how custom images produce apt_sku and how the generated APT sources are used during VM boot. Done means a custom Linux image boots successfully and its APT sources use the image's VERSION_ID when apt_sku is unavailable.