Fix platform check in workflow

.github/workflows/build.yaml

@@ -18,13 +18,13 @@ jobs:
     steps:
       - name: Install LLVM and Clang # required for bindgen to work, see https://github.com/rust-lang/rust-bindgen/issues/1797
         uses: KyleMayes/install-llvm-action@v1
-        if: matrix.config.platform == 'windows-latest'
+        if: matrix.platform == 'windows-latest'
         with:
           version: 12.0.1
           directory: ${{ runner.temp }}/llvm
       - name: Set LIBCLANG_PATH
         run: echo "LIBCLANG_PATH=$((gcm clang).source -replace "clang.exe")" >> $env:GITHUB_ENV
-        if: matrix.config.platform == 'windows-latest'
+        if: matrix.platform == 'windows-latest'
       - uses: actions/checkout@v2
         with:
           submodules: recursive