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 `&&`.

---
[//]: # (BEGIN SAPLING FOOTER)
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* #2467
* __->__ #2465

codex-rs/execpolicy/src/execv_checker.rs

@@ -214,7 +214,12 @@ system_path=[{fake_cp:?}]
 
         // Only readable folders specified.
         assert_eq!(
-            checker.check(valid_exec.clone(), &cwd, &[root_path.clone()], &[]),
+            checker.check(
+                valid_exec.clone(),
+                &cwd,
+                std::slice::from_ref(&root_path),
+                &[]
+            ),
             Err(WriteablePathNotInWriteableFolders {
                 file: dest_path.clone(),
                 folders: vec![]
@@ -226,8 +231,8 @@ system_path=[{fake_cp:?}]
             checker.check(
                 valid_exec.clone(),
                 &cwd,
-                &[root_path.clone()],
-                &[root_path.clone()]
+                std::slice::from_ref(&root_path),
+                std::slice::from_ref(&root_path)
             ),
             Ok(cp.clone()),
         );
@@ -246,8 +251,8 @@ system_path=[{fake_cp:?}]
             checker.check(
                 valid_exec_call_folders_as_args,
                 &cwd,
-                &[root_path.clone()],
-                &[root_path.clone()]
+                std::slice::from_ref(&root_path),
+                std::slice::from_ref(&root_path)
             ),
             Ok(cp.clone()),
         );
@@ -269,8 +274,8 @@ system_path=[{fake_cp:?}]
             checker.check(
                 exec_with_parent_of_readable_folder,
                 &cwd,
-                &[root_path.clone()],
-                &[dest_path.clone()]
+                std::slice::from_ref(&root_path),
+                std::slice::from_ref(&dest_path)
             ),
             Err(ReadablePathNotInReadableFolders {
                 file: root_path.parent().unwrap().to_path_buf(),