contracts-bedrock: fix mainnet deploy-config export

Fixes the export so it works as expected with hardhat deploy config.
Co-authored-by: Andreas Bigger <abigger87@gmail.com>

packages/contracts-bedrock/deploy-config/mainnet.ts

@@ -22,6 +22,5 @@ import mainnetJson from './mainnet.json'
 //
 // The following role is assigned to the Mint Manager contract:
 // - governanceTokenOwner
-const config: DeployConfig = mainnetJson
 
-export default config
+export default mainnetJson satisfies DeployConfig