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

### 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` 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_

Start by tracing the forge library crate's MultiContractRunnerBuilder into the cheatcodes crate configuration. Define how the server feature flag supplies a Hashmap as the injected environment, and verify that untrusted-contract tests can run without reading system environment variables.

Written by the indexing model from the issue text.