CI fix (#47)

use stable toolchain

Author: idea404

.github/workflows/tests.yml

@@ -20,7 +20,7 @@ jobs:
       - name: Install Rust
         uses: actions-rs/toolchain@v1
         with:
-          toolchain: 1.61.0
+          toolchain: stable
           target: wasm32-unknown-unknown
       - name: Build smart contract
         run: npm run build
@@ -42,7 +42,7 @@ jobs:
       - name: Install Rust
         uses: actions-rs/toolchain@v1
         with:
-          toolchain: 1.61.0
+          toolchain: stable
           target: wasm32-unknown-unknown
       - name: Build smart contract
         run: npm run build