Ensure SSZ builder works

[michaelsproul]

@pawanjay176 is investigating issues with our SSZ builder implementation, added in:

• Add builder SSZ flow #6859

There may be no fault in our impl, it could be an issue on the mev-boost side.

Either way, we may want to harden our error handling so that we disable SSZ if we get an error response.

We could set ssz_enabled to result.is_ok() here, rather than setting it to true:

lighthouse/beacon_node/builder_client/src/lib.rs

Lines 135 to 142 in 1888be5

	self.ssz_enabled.store(true, Ordering::SeqCst);
	T::from_ssz_bytes_by_fork(&response_bytes, fork_name)
	.map(|data| ForkVersionedResponse {
	version: Some(fork_name),
	metadata: EmptyMetadata {},
	data,
	})
	.map_err(Error::InvalidSsz)

Similarly, here:

lighthouse/beacon_node/execution_layer/src/lib.rs

Lines 1905 to 1908 in 1888be5

	builder
	.post_builder_blinded_blocks_ssz(block)
	.await
	.map_err(Error::Builder)