Update GitHub Actions workflows: remove redundant branch from docs-and-testcov and enhance Rust setup in run-unit-tests

.github/workflows/run-unit-tests.yml

@@ -39,8 +39,11 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
-      - name: Install Rust
-        run: rustup update stable
+      - name: Set up Rust
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: stable
+          override: true
       - name: Install cargo-llvm-cov
         uses: taiki-e/install-action@cargo-llvm-cov