Use caching instead of artifacts to store .terraform (#2)

Co-authored-by: Ben Martin <ben.martin@sky.uk>
Reviewed-on: #2

modules/dummy/main.tf

@@ -1,4 +1,4 @@
 resource "nomad_job" "dummy" {
   jobspec = file("${path.module}/jobspec.json")
-  json = true
+  json    = true
 }