[experiments] Only create the view if the index must be done

There's no reason to create the view early if it might not
be used.

beat/cmdline/experiments.py

@@ -152,7 +152,6 @@ def run_experiment(
 
     def index_experiment_databases(cache_path, experiment):
         for block_name, infos in experiment.datasets.items():
-            view = infos["database"].view(infos["protocol"], infos["set"])
             filename = toPath(
                 hashDataset(infos["database"].name, infos["protocol"], infos["set"]),
                 suffix=".db",
@@ -163,6 +162,7 @@ def run_experiment(
                     "Index for database %s not found, building it",
                     infos["database"].name,
                 )
+                view = infos["database"].view(infos["protocol"], infos["set"])
                 view.index(database_index_path)
 
     dataformat_cache = {}