Use Alchemy URL from environment

[agostbiro]

Problem

There still some hard-coded Alchemy credentials in the Solidity tests codebase (especially in .sol test files). This is a problem, because these publicly available keys are less reliable than per-user, potentially higher throughput keys. It's not high prio to fix it though as the RPC calls they're used for are heavily cached and we haven't seen any timeouts yet.

Definition of Done

When searching for g.alchemy.com or alchemyapi.io, there are no hard-coded credentials.