fix(.github): modify rust toolchain version.

Rust unstable nightly version causes github action to fail. Fixed it to a stable version.

Author: He1pa

.github/workflows/github-actions.yaml

@@ -30,7 +30,7 @@ jobs:
       - name: Install rust nightly toolchain
         uses: actions-rs/toolchain@v1
         with:
-          toolchain: nightly
+          toolchain: 1.6.1
           override: true
           components: clippy, rustfmt
       - name: Rust grammar test