[landing][keyserver] Allow setting Alchemy key in keyserver/.env

Summary:
This will get downloaded to clients and isn't obfuscated or anything.

We prefer to have clients call Alchemy directly so we don't get any unnecessary user data.

If we end up concerned later that somebody is stealing our API key, we can reset it to another one and roll out some better obfuscation via a Webpack plugin or something.

Depends on D6112

Task is ENG-1282

Test Plan: I tested the Docker keyserver build, setting COMM_ALCHEMY_KEY in keyserver/.env and making sure that the key was set in both server-side and client-side rendering contexts

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6113