feat(forge/cheatcodes): "server" feature flag to disable sys env var reads

[nhtyy]

Component

Forge

Describe the feature you would like

Tracked by #9012

In order to safely run tests on untrusted contracts in the server setting, currently users must fork to disable system env reads.

I propose adding a feature flag to the forge library crate and the cheatcodes crate to use a provided Hashmap<String, String> as an "injected environment".

I think this should be set on the MultiContractRunnerBuilder and then passed to the cheat codes config

Additional context

No response