templates: Decouple the first stage nfs location from baseurl

The baseurl variable is useful to load images to be tested from a non-standard location, for instance when submitting tests from branches that haven't been merged yet and that uploaded their artifacts to https://images.apertis.org/test/ instead of https://images.apertis.org/.

However, to be used in that way the location of the nfs artifacts used in the first stage should not change when setting baseurl.