[Proposal] `leo build` shouldn't display the private key from the `.env` file

[mohammadfawaz]

💥 Proposal

When running leo build with a .env file available, the following is displayed:

📢 Loading environment variables from a `.env` file in the directory tree.
  - NETWORK=testnet
  - PRIVATE_KEY=APrivateKey1zkp8CZNn3yeCseEtxuVPbDCwSyhGW6yZKUYKfgXmcpoGPWH
  - ENDPOINT=https://api.explorer.provable.com/v1
...

it's probably better and more secure if the private key is not actually displayed. Instead, we could display the corresponding public key.

[vicsn]

Instead, we could display the corresponding public key.

This feels like bikeshedding, and may also be confusing, IMO we just use ****** to start