Use network magic to match mainnet, not networkId

[rhyslbw]

Uniqueness for the real mainnet can only be reliably determined from the networkMagic, as despite the property name, networkId is not an identifier but instead it's a type of network. Case in point, the staging network is a mainnet, but has a unique magic.

https://github.com/input-output-hk/cardano-rosetta/blob/2240d9e0840cb96bcc20a5606c0408a46b7b1eef/cardano-rosetta-server/src/server/services/network-service.ts#L43 ... needs to be treated like: https://github.com/input-output-hk/cardano-rosetta/blob/2240d9e0840cb96bcc20a5606c0408a46b7b1eef/cardano-rosetta-server/src/server/services/network-service.ts#L44