Update new end to end test configuration

Author: jpraynaud

mithril-test-lab/mithril-end-to-end/src/mithril/signer.rs

@@ -22,6 +22,7 @@ impl Signer {
             ("RUN_INTERVAL", "2000"),
             ("AGGREGATOR_ENDPOINT", &aggregator_endpoint),
             ("DB_DIRECTORY", db_directory.to_str().unwrap()),
+            ("STAKE_STORE_DIRECTORY", "./store/stakes")
         ]);
         let args = vec!["-vvv"];