Read testnet wallets' private keys from environment variables

[digorithm]

To avoid leaking testnet wallets' private keys, we shouldn't hardcode these wallets' private keys like we do, e.g., in here:

https://github.com/FuelLabs/fuels-rs/blob/master/packages/fuels/tests/providers.rs#L597

Instead, we should read these private keys from environment variables and set them in the GitHub CI environment. Similar to what was done recently on the TS SDK: FuelLabs/fuels-ts#1858. We should generate a new test wallet, fund it through the faucet, and then set the env var on GitHub.

[arboleya]

Maybe not relevant here, but as we also had references to WALLET_ADDRESS, we started reading it from the environment/secrets as well. If we need to change things in the future, no code modifications will be necessary.

[digorithm]

Maybe not relevant here, but as we also had references to WALLET_ADDRESS, we started reading it from the environment/secrets as well. If we need to change things in the future, no code modifications will be necessary.

💯 we should do the same here.

[Br1ght0ne]

The linked test was removed in #1245. We'll need to use envvars next time we add a test like this.