(air gapped): disable fluxcd artifact by default (#964)

Default `artifact` field for FluxCD operator to empty string, so it uses embedded manifests and is compatible with air-gapped environments.

packages/system/fluxcd/values.yaml

@@ -4,6 +4,7 @@ flux-instance:
       networkPolicy: true
       domain: cozy.local # -- default value is overriden in patches
     distribution:
+      artifact: ""
       version: 2.5.x
       registry: ghcr.io/fluxcd
     components: