schemas/osbuild1: specify exact schema version

Using the "$schema" without any specific version has been
deprecated[1] and will lead to warnings like:

  DeprecationWarning: The metaschema specified by
  $schema was not found. Using the latest draft to validate,
  but this will raise an error in the future

Fix this by pointing $schema to "draft-07", which is the latest
version fully supported the jsonschema python package, that is
being used internally.

[1]
  "The possibility to declare $schema without specific version
  (http://json-schema.org/schema#) was deprecated after Draft 4
  and should no longer be used."
  https://json-schema.org/understanding-json-schema/reference/schema.html

schemas/osbuild1.json

@@ -1,5 +1,5 @@
 {
-  "$schema": "https://json-schema.org/schema",
+  "$schema": "http://json-schema.org/draft-07/schema#",
   "$id": "https://osbuild.org/schemas/osbuild1.json",
 
   "title": "OSBuild Manifest",