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.rs

@@ -349,14 +349,12 @@ pub(crate) async fn consume_truncated_output(
     // we treat it as an exceptional I/O error
 
     let stdout_reader = child.stdout.take().ok_or_else(|| {
-        CodexErr::Io(io::Error::new(
-            io::ErrorKind::Other,
+        CodexErr::Io(io::Error::other(
             "stdout pipe was unexpectedly not available",
         ))
     })?;
     let stderr_reader = child.stderr.take().ok_or_else(|| {
-        CodexErr::Io(io::Error::new(
-            io::ErrorKind::Other,
+        CodexErr::Io(io::Error::other(
             "stderr pipe was unexpectedly not available",
         ))
     })?;