Add download test

[isidroamv]

This PR solves part of the issues reported in the issue-9:

Changes:

• Moves alchemy key into secrets
• Adds the test for download functionality into the CI

CC: @Divide-By-0 , @saleel

[render]

Your Render PR Server URL is https://proof-of-twitter-pr-11.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-cmvhdm0l6cac73foe3h0.

[saleel]

Thank you.

I have been trying this, and the Job was getting timed out even though the log says it finished all steps - https://github.com/zkemail/proof-of-twitter/actions/runs/7728773573/job/21075860938

Looks like the trick is to set the args and executablePath like you did.