mkpasswd: Update to 5.0.26

The previous version (5.0.26) has been removed from the debian ftp.
As the source URL is now down, our own hydra (not hydra.nixos.org) failed
to build the package.
This problem will occur again in the future since I only updated the URL
without relying on a "more stable" alternative (this merits a specific
discussion).
Merge pull request #993 from bbenoist/mkpasswd-fix

mkpasswd: Update to 5.0.26