CI: correct workflow definition

Author: compnerd

.github/workflows/build.yml

@@ -2,10 +2,13 @@ name: build
 
 on:
   pull_request:
+    branches: [ 'main' ]
   workflow_dispatch:
 
 jobs:
   windows:
+    runs-on: windows-latest
+
     strategy:
       matrix:
         include:
@@ -42,8 +45,14 @@ jobs:
 
       - uses: actions/checkout@v2
 
+      # - uses: seanmiddleditch/gha-setup-ninja@master
+      - uses: seanmiddleditch/gha-setup-vsdevenv@master
+
       - name: configure
-        run: cmake -B out -D CMAKE_BUILD_TYPE=Release -G Ninja -S ${{ github.workspace }}
+        run: |
+          # workaround CMake 3.20.0 bug
+          $SDK_ROOT = cygpath -m $env:SDKROOT
+          cmake -B out -D CMAKE_BUILD_TYPE=Release -D CMAKE_C_COMPILER=clang-cl -D CMAKE_CXX_COMPILER=clang-cl -D CMAKE_MT=mt -D CMAKE_Swift_FLAGS="-sdk $SDK_ROOT" -G Ninja -S ${{ github.workspace }}
 
       - name: build
-        run: cmake --build out --configuration Release
+        run: cmake --build out --config Release