fix(reth_entrypoint): add flashblocks to additional args instead of rewritting

[mur-me]

Today I found an issue with additional args in reth entrypoint, one of my args want propagate to the reth.

I've checked the code and found the root cause, ADDITIONAL_ARGS="--websocket-url=$RETH_FB_WEBSOCKET_URL" is completely rewriting the ADDITIONAL_ARGS value.

Funny, that pruning agrs don't have this issue

What was done:

• add append instead of rewritting
• add explicit echo

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	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 0 Sum 1