tools/terraform: Fix warning reported by Terraform

We shouldn't interpolate a variable alone in a string: we should use the
variable directly.

Author: dumbbell

deps/rabbitmq_ct_helpers/tools/terraform/direct-vms/main.tf

@@ -212,7 +212,7 @@ resource "aws_instance" "vm" {
   connection {
     type        = "ssh"
     user        = local.username
-    private_key = file("${var.ssh_key}")
+    private_key = file(var.ssh_key)
     agent       = false
   }
 }