refactor: skylab user ssh pub key as var

opdracht-3/variables.tf

@@ -25,3 +25,7 @@ variable "prefix" {
   description = "The Prefix used for all resources"
   default     = "week-5-opdracht-3"
 }
+
+variable "skylab_ssh_public_key" {
+  description = "Public key of the Skylab user"
+}