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/Cargo.toml

@@ -19,6 +19,7 @@ client.workspace = true
 collections.workspace = true
 context_server.workspace = true
 dirs = "5.0"
+dotenv.workspace = true
 env_logger.workspace = true
 extension.workspace = true
 fs.workspace = true

crates/eval/README.md

@@ -6,6 +6,8 @@ This eval assumes the working directory is the root of the repository. Run it wi
 cargo run -p eval
 ```
 
+The eval will optionally read a `.env` file in `crates/eval` if you need it to set environment variables, such as API keys.
+
 ## Explorer Tool
 
 The explorer tool generates a self-contained HTML view from one or more thread

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();