Update Rust toolchain step on CI

Author: 0xPxt

.github/workflows/main.yaml

@@ -20,6 +20,10 @@ jobs:
         uses: actions/checkout@v2
         with:
           submodules: true
+      - name: Update Rust toolchain
+        run: |
+          rustup update stable
+          rustup default stable
       - name: Install Miri
         run: |
           rustup toolchain install nightly --component miri