Add distinct env var for Covenant Network in the config

[dbkbali]

• needed to amend config to account for the different naming convention for btc mainnet
• btc uses "main" as the designated main network for bitcoin and not "mainnet"
• the covenant signer uses "mainnet" as the designation for operating on the btc main network as a result we have added a env var to our jobs distinct from BTC_CHAIN which allows the covenant signer network to be set to "mainnet" or another covenant signer network