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.

.github/workflows/rust-release.yml

@@ -74,7 +74,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
-      - uses: dtolnay/rust-toolchain@stable
+      - uses: dtolnay/rust-toolchain@1.86
         with:
           targets: ${{ matrix.target }}