fix: public load_auth() fn always called with include_env_var=true (#1961)

Apparently `include_env_var=false` was only used for testing, so clean
up the API a little to make that clear.


---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/1961).
* #1962
* __->__ #1961

codex-rs/cli/src/proto.rs

@@ -36,7 +36,7 @@ pub async fn run_main(opts: ProtoCli) -> anyhow::Result<()> {
         .map_err(anyhow::Error::msg)?;
 
     let config = Config::load_with_cli_overrides(overrides_vec, ConfigOverrides::default())?;
-    let auth = load_auth(&config.codex_home, true)?;
+    let auth = load_auth(&config.codex_home)?;
     let ctrl_c = notify_on_sigint();
     let CodexSpawnOk { codex, .. } = Codex::spawn(config, auth, ctrl_c.clone()).await?;
     let codex = Arc::new(codex);