chore: upgrade to Rust 1.89 (#2465)

Codex created this PR from the following prompt:

> upgrade this entire repo to Rust 1.89. Note that this requires
updating codex-rs/rust-toolchain.toml as well as the workflows in
.github/. Make sure that things are "clippy clean" as this change will
likely uncover new Clippy errors. `just fmt` and `cargo clippy --tests`
are sufficient to check for correctness

Note this modifies a lot of lines because it folds nested `if`
statements using `&&`.

---
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* #2467
* __->__ #2465

codex-rs/arg0/src/lib.rs

@@ -89,10 +89,10 @@ const ILLEGAL_ENV_VAR_PREFIX: &str = "CODEX_";
 /// Security: Do not allow `.env` files to create or modify any variables
 /// with names starting with `CODEX_`.
 fn load_dotenv() {
-    if let Ok(codex_home) = codex_core::config::find_codex_home() {
-        if let Ok(iter) = dotenvy::from_path_iter(codex_home.join(".env")) {
-            set_filtered(iter);
-        }
+    if let Ok(codex_home) = codex_core::config::find_codex_home()
+        && let Ok(iter) = dotenvy::from_path_iter(codex_home.join(".env"))
+    {
+        set_filtered(iter);
     }
 
     if let Ok(iter) = dotenvy::dotenv_iter() {