op-devstack: use file-based sequencer key loading for kona

[DenseDenise]

kona now supports loading sequencer keys from files (op-rs/kona#2871), switched from KONA_NODE_P2P_SEQUENCER_KEY to KONA_NODE_P2P_SEQUENCER_KEY_PATH to match the pattern used for regular p2p keys

[wiz-inc-a178a98b5d[bot]]

Wiz Scan Summary

Scanner	Findings
Vulnerabilities	-
Sensitive Data	-
Secrets	-
IaC Misconfigurations	-
SAST Findings	1
Total	1

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