Properly sign the unresolved drvs

Don't let them inherit the signature from the parent one (because it
makes no sense to do so), but re-sign them after they have been built

src/libstore/build/derivation-goal.hh

@@ -180,6 +180,9 @@ struct DerivationGoal : public Goal
     /* Open a log file and a pipe to it. */
     Path openLogFile();
 
+    /* Sign the newly built realisation if the store allows it */
+    virtual void signRealisation(Realisation&) {}
+
     /* Close the log file. */
     void closeLogFile();