ci: use specific toolchain version 1.81.0 in tests instead of stable

Author: denbite

.github/workflows/tests.yml

@@ -22,7 +22,7 @@ jobs:
       uses: actions-rs/toolchain@v1
       with:
         profile: minimal
-        toolchain: stable
+        toolchain: 1.81.0
         default: true
         target: wasm32-unknown-unknown