Check for derivation before creating build products.

It happens that we receive build succeeded notifications for derivations that
are not stored in the Builds table. In that case, do not try to create build
products.

* src/cuirass/base.scm (handle-build-event): Check if the derivation is
registered in the Builds table.

src/cuirass/base.scm

@@ -577,9 +577,10 @@ updating the database accordingly."
      (if (valid? drv)
          (begin
            (log-message "build succeeded: '~a'" drv)
-           (when spec
-             (create-build-outputs (db-get-build drv)
-                                   (assq-ref spec #:build-outputs)))
+           (let ((build (db-get-build drv)))
+             (when (and spec build)
+               (create-build-outputs build
+                                     (assq-ref spec #:build-outputs))))
            (db-update-build-status! drv (build-status succeeded))
 
            (for-each (match-lambda