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

crates/eval/src/eval.rs

@@ -34,9 +34,12 @@ use std::collections::VecDeque;
 use std::env;
 use std::path::{Path, PathBuf};
 use std::rc::Rc;
-use std::sync::Arc;
+use std::sync::{Arc, LazyLock};
 use util::ResultExt as _;
 
+static CARGO_MANIFEST_DIR: LazyLock<PathBuf> =
+    LazyLock::new(|| PathBuf::from(env!("CARGO_MANIFEST_DIR")));
+
 #[derive(Parser, Debug)]
 #[command(name = "eval", disable_version_flag = true)]
 struct Args {
@@ -57,6 +60,8 @@ struct Args {
 }
 
 fn main() {
+    dotenv::from_filename(CARGO_MANIFEST_DIR.join(".env")).ok();
+
     env_logger::init();
 
     let system_id = ids::get_or_create_id(&ids::eval_system_id_path()).ok();