fix: update nethermind config option (#824)

Fixes config file name for Nethermind:
- All *.cfg files were renamed to *.json as part of this PR:
https://github.com/NethermindEth/nethermind/pull/7653
- Skipping file extension will automatically add a proper one
Signed-off-by: Alex <alexb5dh@gmail.com>

src/el/nethermind/nethermind_launcher.star

@@ -159,7 +159,7 @@ def get_config(
         cmd.append("--config=" + launcher.network.split("-")[0])
         cmd.append("--Init.BaseDbPath=" + launcher.network.split("-")[0])
     elif launcher.network not in constants.PUBLIC_NETWORKS:
-        cmd.append("--config=none.cfg")
+        cmd.append("--config=none")
         cmd.append(
             "--Init.ChainSpecPath="
             + constants.GENESIS_CONFIG_MOUNT_PATH_ON_CONTAINER