Avoid changing secrets in e2e tests

The https://github.com/api3dao/signed-api/blob/33e59702d55a0a9033d5a861f62bf4efa89cffbf/packages/e2e/src/pusher/secrets.env#L3C1-L3C1 is pretty annoying and error prone. It would be nice to just set it once.

We can make the file an example and let the user create a new one with correct ENV value set. This will also fail, when user does not create the file.