Fix job insertion with existing outputs.

Different derivations may result in a same build output. If a job registers a
derivation D2 with the output O1, Cuirass may already have processed another
derivation D1 with the same O1 output.

In that case, the new job must point to the D1 derivation and not the D2
derivation, for which no build will be triggered.

* src/cuirass/database.scm (db-add-job): Fix it.
* tests/database.scm ("db-register-builds same-outputs", "db-get-jobs
same-outputs"): New tests.