When testing changes to https://github.com/viabtc/viabtc_mining_server I noticed that `-usecashaddr=0` did
not impact the address format for the miner fund in `getblocktemplate`. This patch fixes that.
While it would be ideal to move away from legacy address support entirely, it's still very common that mining pool software only support the legacy format. Some reasons for why this is important:
1. Pushing BCH-prefixed cashaddr on pools will cause disruption and confusion since we expect a rebranded prefix to be pushed later.
2. Pools often mine to the same address across different chains, so supporting legacy addresses for the time being is important to them to prevent over-complicating their pool configurations.