Fix transition from declarative non-flake to flake jobset

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>

Created by  Silvan Mosberger  on February 1, 2021
7QEA2OA4DQPRPG7PYHNIHBIIK3AJLUPKD7I7XXNOGII2MYD62LFAC
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