Fix Lambda S3 bucket cross-region access error (#9)

* Fix Lambda S3 bucket cross-region access error

The agent-scaler Lambda was using a hardcoded "buildkite-lambdas"
bucket, causing PermanentRedirect errors in regions outside us-east-1.
Now uses region-specific buckets (e.g., buildkite-lambdas-us-west-2),
incorporating the mapping from agent-scaler's SAM template
(https://github.com/buildkite/buildkite-agent-scaler/blob/ac0883ab632a8689384ec8991c6415f8f3b48052/template.yaml#L166-L187).

* Use ternary to construct bucket name instead of relying on mapping

Author: scadu

agent-scaler.tf

@@ -5,7 +5,7 @@ resource "aws_lambda_function" "scaler" {
   function_name = "${local.stack_name_full}-scaler"
   description   = "Scales ${aws_autoscaling_group.agent_auto_scale_group.name} based on Buildkite metrics"
 
-  s3_bucket = "buildkite-lambdas"
+  s3_bucket = local.agent_scaler_s3_bucket
   s3_key    = "buildkite-agent-scaler/v${var.buildkite_agent_scaler_version}/handler.zip"
 
   handler       = "bootstrap"

locals.tf

@@ -63,6 +63,10 @@ locals {
     cloudformation_stack_version = "v6.46.0"
   }
 
+  # Region-specific Lambda deployment bucket
+  # us-east-1 uses "buildkite-lambdas", all other regions append the region suffix
+  agent_scaler_s3_bucket = data.aws_region.current.id == "us-east-1" ? "buildkite-lambdas" : "buildkite-lambdas-${data.aws_region.current.id}"
+
   # Detect ARM and burstable instances from instance type family
   instance_type_family = split(".", split(",", var.instance_types)[0])[0]