bug: .env.example is incorrect for hardhat example

[bajpai244]

We have a .env.example, where we are using:

NIL_WALLET_ADDR="....."

But, if we look at the config file for hardhat, it accepts WALLET_ADDR instead; hence, any running hardhat ignition will fail because of the wrong environment.

Another issue is the formatting of the .env.example file where in the first two cases, we have used ENV_NAME: VALUE format, while in the last one, we have used ENV_NAME=VALUE, we should aim for consistency!