http: Add evaluation field to hydra-build.

* src/cuirass/http.scm (build->hydra-build): Add "evaluation" field.

src/cuirass/http.scm

@@ -87,6 +87,7 @@
                      (build-status started)))))
 
   `((#:id . ,(assq-ref build #:id))
+    (#:evaluation . ,(assq-ref build #:eval-id))
     (#:jobset . ,(assq-ref build #:specification))
     (#:job . ,(assq-ref build #:job-name))