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.


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* #1962
* __->__ #1961

codex-rs/core/src/codex_wrapper.rs

@@ -26,7 +26,7 @@ pub struct CodexConversation {
 /// that callers can surface the information to the UI.
 pub async fn init_codex(config: Config) -> anyhow::Result<CodexConversation> {
     let ctrl_c = notify_on_sigint();
-    let auth = load_auth(&config.codex_home, true)?;
+    let auth = load_auth(&config.codex_home)?;
     let CodexSpawnOk {
         codex,
         init_id,