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]

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[haardikk21]

This seems good. Could you rebase?

[mur-me]

@haardikk21

This seems good. Could you rebase?

Done, thanks