The database has these constraints:
check ((type = 0) = (nixExprInput is not null and nixExprPath is not null)),
check ((type = 1) = (flake is not null)),
which prevented switching to flakes in a declarative jobspec, since the
nixexpr{path,input} fields were not nulled in such an update
Co-Authored-By: Graham Christensen <graham@grahamc.com>