fix(ci): configure windows Rust

Add env vars for libclang and use the gnu toolchain.

Author: zah

.agents/tasks/2025/06/29-2319-ci-failure-analysis

@@ -0,0 +1 @@
+Navigate to the GitHub CI runs and examine the Windows failures, try to guess the problem and propose a fix

.github/workflows/ci.yml

@@ -31,6 +31,8 @@ jobs:
           elif [[ "$RUNNER_OS" == "Windows" ]]; then
             # Install LLVM/Clang for Windows
             choco install llvm -y
+            echo "LIBCLANG_PATH=C:\\Program Files\\LLVM\\bin" >> $GITHUB_ENV
+            echo "CLANG_PATH=C:\\Program Files\\LLVM\\bin\\clang.exe" >> $GITHUB_ENV
             # Install vcpkg for Cap'n Proto on Windows
             git clone https://github.com/Microsoft/vcpkg.git
             cd vcpkg
@@ -47,6 +49,8 @@ jobs:
           ruby-version: '3.2'
       - name: Setup Rust
         uses: dtolnay/rust-toolchain@stable
+        with:
+          toolchain: ${{ matrix.os == 'windows-latest' && 'stable-x86_64-pc-windows-gnu' || 'stable' }}
       - name: Install Ruby dependencies
         run: bundle install
       - name: Setup just