Rename Index env vars

[josecelano]

Related to: torrust/torrust-index#596

Index env vars have been renamed. We have to update them here.

TORRUST_INDEX_CONFIG             -> TORRUST_INDEX_CONFIG_TOML
TORRUST_INDEX_PATH_CONFIG        -> TORRUST_INDEX_CONFIG_TOML_PATH
TORRUST_INDEX_TRACKER_API_TOKEN  -> TORRUST_INDEX_CONFIG_OVERRIDE_TRACKER__TOKEN
TORRUST_INDEX_AUTH_SECRET_KEY    -> TORRUST_INDEX_CONFIG_OVERRIDE_AUTH__SECRET_KEY