chore(ci): switch testing to stable Rust

.github/workflows/rust.yaml

@@ -15,10 +15,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - name: Install latest nightly
+      - name: Install latest stable
         uses: actions-rs/toolchain@v1
         with:
-          toolchain: nightly
+          toolchain: stable
           override: true
           components: rustfmt, clippy