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

.gitignore

@@ -32,4 +32,5 @@ Packages
 xcuserdata/
 
 # Don't commit any secrets to the repo.
+.env
 .env.secret.toml