fix(nethermind): validate OP_NODE_NETWORK in entrypoint

[HoustonOla35]

Description

Replace unused NETWORK check to match --config="$OP_NODE_NETWORK".
Prevents false exits; no behavior change for correct configs. Verified with/without OP_NODE_NETWORK.

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2

[danyalprout]

lgtm ty