ci: use dtolnay/rust-toolchain, the action that exists

dtolnay/rust-action does not exist, so the test job failed at setup
("Unable to resolve action dtolnay/rust-action, repository not found")
before running a single Rust test.

.github/workflows/ci.yml

@@ -39,7 +39,7 @@ jobs:
           cache: 'yarn'
 
       - name: Setup Rust
-        uses: dtolnay/rust-action@stable
+        uses: dtolnay/rust-toolchain@stable
 
       - name: Install dependencies
         run: yarn install --immutable