stages/lorax-script: add missing default basearch to the schema

The code uses x86_64 as a default, see:

basearch = options.get("basearch", "x86_64")

Let's declare that explicitly in the schema to prevent any confusion.

stages/org.osbuild.lorax-script

@@ -32,6 +32,7 @@ SCHEMA = """
   },
   "basearch": {
     "type": "string",
+    "default": "x86_64",
     "description": "The basic architecture param to supply to the template"
   },
   "product": {