new tar source etag saved strangely

When running bst track on tar sources, we save something like this:

  etag: '"2c6740-53c8cf064bf9d"'

This seems quite odd; I would expect something like:

  etag: 2c6740-53c8cf064bf9d

@juergbi is this intentional for some reason ?

Perhaps this is a subtle regression from fixing the loader to load everything as strings ?