ci: update toolchain version to most recent stable

Author: henrybarreto

.github/workflows/code_style.yml

@@ -16,7 +16,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: nightly
+          toolchain: stable
           override: true
           components: rustfmt, clippy