set libclang path on windows

see rust-lang/rust-bindgen#1797

Author: lovasoa

.github/workflows/rust.yml

@@ -19,6 +19,9 @@ jobs:
           - os: macos-latest
           - os: windows-latest
     steps:
+    - name: Set LIBCLANG_PATH
+      run: echo "LIBCLANG_PATH=$((gcm clang).source -replace "clang.exe")" >> $env:GITHUB_ENV
+      if: matrix.os == "windows-latest"
     - uses: actions/checkout@v2
       with:
         submodules: recursive