eval: Add support for reading from a .env file (#29426)

This PR adds support for the eval to read environment variables from a
`.env` file located in the `crates/eval` directory.

For instance, you can use it to set your Anthropic API key:

```
ANTHROPIC_API_KEY=<secret>
```

Release Notes:

- N/A

Cargo.toml

@@ -437,6 +437,7 @@ dashmap = "6.0"
 dap-types = { git = "https://github.com/zed-industries/dap-types", rev = "be69a016ba710191b9fdded28c8b042af4b617f7" }
 derive_more = "0.99.17"
 dirs = "4.0"
+dotenv = "0.15.0"
 ec4rs = "1.1"
 emojis = "0.6.1"
 env_logger = "0.11"