Add ETHERSCAN_TOKEN secret

[Veremeenko Artyom]

Due to the Etherscan API limit:

example run

BrownieEnvironmentWarning: No etherscan API token set. You may experience issues with rate limiting. Visit https://etherscan.io/register to obtain a token, and then store it as the environment variable $ETHERSCAN_TOKEN

ValueError: Failed to retrieve data from API: {'status': '0', 'message': 'NOTOK', 'result': 'Max rate limit reached, please use API Key for higher rate limit'}

[Eugene Mamin]

I see that we have
ETHERSCAN_TOKEN: ${{ secrets.ETHERSCAN_TOKEN }}
for our CI workflow setup

CC: @n0guest, @dgusakov, could you please confirm that setup is correct?

[EK]

Sorry, my bad (mistype in secret's name). Just fixed, plz, try again

[Eugene Mamin]

Resolved by @n0guest