As a work of the US government, there isn't any license appropriate for the work, because it's already in the public domain (in the United States). So a license like the Unlicense (or CC0), in which the licensor is entering the covered work into the public domain, doesn't work. Some text that acknowledges the public domain status in the US is helpful (and desirable), but it's not a "license".
The above situation only applies to the US, unfortunately -- government works are potentially copyrightable in non-US contexts. So, some formal text or license that enters the work into the worldwide public domain is appropriate (and desirable).
My favorite example of this is what HHS has done on their ckanext-datajson extension:
As a work of the United States Government, this package is in the public domain within the United States. Additionally, we waive copyright and related rights in the work worldwide through the CC0 1.0 Universal public domain dedication (which can be found at http://creativecommons.org/publicdomain/zero/1.0/).