Resolve issue with env variable being overwritten

CI set the environment variable on build
`DAPP_DEVELOPMENT_TESTNET_CONTRACTS` but the dapp didn't read that because
it had `REACT_APP_DAPP_DEVELOPMENT_TESTNET_CONTRACTS = false` and `.env`
took precedence. The change sets the `.env` based on the specified
environment variable so the dapp will now take that into account.

.env

@@ -2,7 +2,7 @@ REACT_APP_SUPPORTED_CHAIN_ID=1337
 REACT_APP_ETH_HOSTNAME_HTTP=http://localhost:8545
 REACT_APP_ETH_HOSTNAME_WS=ws://localhost:8545
 REACT_APP_MULTICALL_ADDRESS=$MULTICALL_ADDRESS
-REACT_APP_DAPP_DEVELOPMENT_TESTNET_CONTRACTS=false
+REACT_APP_DAPP_DEVELOPMENT_TESTNET_CONTRACTS=$DAPP_DEVELOPMENT_TESTNET_CONTRACTS
 
 REACT_APP_FEATURE_FLAG_TBTC_V2=true
 REACT_APP_FEATURE_FLAG_TBTC_V2_REDEMPTION=true

.env.production

@@ -1,6 +1,7 @@
 REACT_APP_SUPPORTED_CHAIN_ID=$CHAIN_ID
 REACT_APP_ETH_HOSTNAME_HTTP=$ETH_HOSTNAME_HTTP
 REACT_APP_ETH_HOSTNAME_WS=$ETH_HOSTNAME_WS
+REACT_APP_DAPP_DEVELOPMENT_TESTNET_CONTRACTS=$DAPP_DEVELOPMENT_TESTNET_CONTRACTS
 
 REACT_APP_FEATURE_FLAG_TBTC_V2=true
 REACT_APP_FEATURE_FLAG_TBTC_V2_REDEMPTION=true