ci: explicitly set up stable Rust toolchain

.github/workflows/ci.yml

@@ -69,6 +69,7 @@ jobs:
     - uses: actions/checkout@v4
       with:
          submodules: true
+    - uses: dtolnay/rust-toolchain@stable
     - uses: mkroening/rust-toolchain-toml@main
     - name: Download loader  
       run: gh release download --repo hermit-os/loader --pattern hermit-loader-x86_64