ci: set Rust toolchain version dynamically in coverage workflow

Update the coverage-improvement GitHub Actions workflow to read the Rust toolchain version
from rust-toolchain.toml and use it for the Rust setup step. This ensures the workflow uses
the exact Rust version specified in the project configuration, improving consistency and
reducing maintenance overhead.

Signed-off-by: Peng Tao <bergwolf@hyper.sh>

Author: bergwolf

.github/workflows/coverage-improvement.yml

@@ -45,10 +45,18 @@ jobs:
         with:
           fetch-depth: 0
 
+      - name: Read Rust toolchain version
+        id: set_toolchain_version
+        run: |
+          RUST_TOOLCHAIN_VERSION=$(grep -oP '(?<=channel = ")[^"]*' rust-toolchain.toml)
+          echo "Rust toolchain version: $RUST_TOOLCHAIN_VERSION"
+          echo "rust-version=$RUST_TOOLCHAIN_VERSION" >> $GITHUB_OUTPUT
+
+      shell: bash
       - name: Setup Rust
         uses: actions-rust-lang/setup-rust-toolchain@v1
         with:
-          toolchain: stable
+          toolchain: ${{ steps.set_toolchain_version.outputs.rust-version }}
           components: llvm-tools-preview
 
       - name: Install cargo-llvm-cov