Disable prefetch on env-computed fields

As in the inverse field that write the value into the <field>_env_default
we have to browse the record, the prefetch has the effect of calling
compute on the env-computed field which resets the value to it's
previous state before we have the occasion to store it.

server_environment/models/server_env_mixin.py

@@ -258,6 +258,7 @@ class ServerEnvMixin(models.AbstractModel):
         field.required = False
         field.copy = False
         field.sparse = None
+        field.prefetch = False
 
     def _server_env_add_is_editable_field(self, base_field):
         """Add a field indicating if we can edit the env-computed fields