Make nethermind entrypoint executable

[dguenther]

Fixes node-nethermind erroring with spawnerr: command at '/app/execution-entrypoint' is not executable under supervisord (mentioned in #429).

Testing

• ✅ Built and exported image with the fix on my fork and started up the arm64 build for ~30 secs.

[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 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 0 Sum 1