refactor: improve robustness of genesis file loading

[barajeel]

This PR:

• Ensures proper checking of ESPRESSO_SEQUENCER_GENESIS_SECRET using [[ -n "${ESPRESSO_SEQUENCER_GENESIS_SECRET:-}" ]].
• Wraps variable references in quotes to handle spaces and special characters correctly.
• Removes explicit error handling for aws secretsmanager, relying on set -e to stop execution on failure.
• Uses exec /bin/sequencer "$@" to replace the shell process instead of spawning a new one.

Key places to review:

• The conditional check for ESPRESSO_SEQUENCER_GENESIS_SECRET.
• Handling of aws secretsmanager output.
• Replacement of /bin/sequencer "$@" with exec /bin/sequencer "$@".