Close eventstreams upon completion

Previously we were not closing eventSource's upon completion which
lead to stale eventstream calls persisting across a session in the
scaffolder. This calls eventSource.close() properly in order to
clean up an eventStream once we get completion event

Signed-off-by: jrusso1020 <jrusso@brex.com>

.changeset/tame-masks-juggle.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-scaffolder': patch
+---
+
+Close eventSource upon completion of a scaffolder task

plugins/scaffolder/src/api.ts

@@ -214,6 +214,7 @@ export class ScaffolderClient implements ScaffolderApi {
                 subscriber.error(ex);
               }
             }
+            eventSource.close();
             subscriber.complete();
           });
           eventSource.addEventListener('error', event => {