Explicitly install a Rust toolchain for cargo-outdated job

Debugging a recent cargo-outdated bug, it would have been nice not to
wonder whether a rustc version change in GitHub's runner image was a
contributing factor.

Author: dtolnay

.github/workflows/ci.yml

@@ -101,5 +101,6 @@ jobs:
     timeout-minutes: 45
     steps:
       - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
       - uses: dtolnay/install@cargo-outdated
       - run: cargo outdated --workspace --exit-code 1