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/core/src/config.rs

@@ -759,10 +759,10 @@ fn default_model() -> String {
 pub fn find_codex_home() -> std::io::Result<PathBuf> {
     // Honor the `CODEX_HOME` environment variable when it is set to allow users
     // (and tests) to override the default location.
-    if let Ok(val) = std::env::var("CODEX_HOME") {
-        if !val.is_empty() {
-            return PathBuf::from(val).canonicalize();
-        }
+    if let Ok(val) = std::env::var("CODEX_HOME")
+        && !val.is_empty()
+    {
+        return PathBuf::from(val).canonicalize();
     }
 
     let mut p = home_dir().ok_or_else(|| {