bug#32331: Cuirass: Restarting Cuirass get it to build a huge lot of duplicate derivations

[Clément Lassieur]

That's because DB-GET-PENDING-DERIVATIONS tries to select *distinct*
derivations but it doesn't work because they all have a distinct id even
though they are the same.

That's a direct consequence of https://bugs.gnu.org/32190.