fix(runners): correct regex pattern for extracting AMI ID from SSM parameter ARN (#4981)

This PR creates a small change to the regex group that ensures the
correct ami_id_ssm_parameter_arn value is passed in its fullest to the
env of the scale-up lambda. At the moment, this is missing the leading
'/' and thus causes ```Error processing batch (size: 3): Failed to
lookup runner AMI ID from SSM parameter:
github-action-runners/staging-multi/<redacted>-ubuntu-x64/runners/config/ami_id,\n
GetParameterError: User:
arn:aws:sts::<redacted>:assumed-role/staging-multi-<redacted>-ubu-x64-med-scale-up-lambda-<redacted>/staging-multi-<redacted>-ubu-x64-med-scale-up
is not authorized to perform: ssm:GetParameter on resource:
arn:aws:ssm:<redacted>:<redacted>:* because no identity-based policy
allows the ssm:GetParameter action, ignoring batch",```.

closes #4959

Co-authored-by: Niek Palm <[email protected]>

Author: stuartp44

modules/runners/main.tf

@@ -48,7 +48,7 @@ locals {
   ami_filter                = merge(local.default_ami[var.runner_os], local.ami_config.filter)
   ami_id_ssm_module_managed = local.ami_config.id_ssm_parameter_arn == null
   # Extract parameter name from ARN (format: arn:aws:ssm:region:account:parameter/path/to/param)
-  ami_id_ssm_parameter_name = local.ami_id_ssm_module_managed ? null : try(regex("parameter/(.+)$", local.ami_config.id_ssm_parameter_arn)[0], null)
+  ami_id_ssm_parameter_name = local.ami_id_ssm_module_managed ? null : try(regex("parameter(/.+)$", local.ami_config.id_ssm_parameter_arn)[0], null)
 
   enable_job_queued_check = var.enable_job_queued_check == null ? !var.enable_ephemeral_runners : var.enable_job_queued_check