Make independent installers for mac/windows nodes: mainnet/testnet (stagenet)

It would be useful to split the mac and windows node installers into two since the cadence of releases on testnet and mainnet always will be out of sync. (Currently, both the mainnet and testnet nodes are installed by the same installer.) This would also make it easier to make a dedicated stagenet node, so that people don't have to reconfigure a testnet/mainnet node manually.

- Change the macOS installer, so it only installs a single net (mainnet/testnet/stagenet/protonet)
- Change the Windows installer, so it only installs a single net (mainnet/testnet/stagenet/protonet)