test: adjust E2E test to be reproducible by external contributors

[mfw78]

Problem

Currently the E2E test makes use of a private key placed in an environment variable in order to make a "live" order post to CoW Protocol, followed by a cancellation. Given that the environment variable isn't injected in the CI/CD for PRs originating from external forks, the CI/CD always fails.

Proposed solution

There are a couple of solutions that may be used:

1. Using CI/CD, spin the services and a forked blockchain, similar to that done in E2E tests within https://github.com/cowprotocol/services.
2. Investigate the minimum requirements for limit order posting as a limit order can be posted without having the sellToken, but may require some dust. In this event, if the dust requirement is so low that it is not economically viable for an adversary in possession of the private key to take the funds, this may be an option.