chore: pin Rust version to 1.86 and use io::Error::other to prepare for 1.87 (#947)

Previously, our GitHub actions specified the Rust toolchain as
`dtolnay/rust-toolchain@stable`, which meant the version could change
out from under us. In this case, the move from 1.86 to 1.87 introduced
new clippy warnings, causing build failures.

Because it will take a little time to fix all the new clippy warnings,
this PR pins things to 1.86 for now to unbreak the build.

It also replaces `io::Error::new(io::ErrorKind::Other)` with
`io::Error::other()` in preparation for 1.87.

codex-rs/core/src/exec_linux.rs

@@ -51,10 +51,9 @@ pub fn exec_linux(
     match tool_call_output {
         Ok(Ok(output)) => Ok(output),
         Ok(Err(e)) => Err(e),
-        Err(e) => Err(CodexErr::Io(io::Error::new(
-            io::ErrorKind::Other,
-            format!("thread join failed: {e:?}"),
-        ))),
+        Err(e) => Err(CodexErr::Io(io::Error::other(format!(
+            "thread join failed: {e:?}"
+        )))),
     }
 }