windows: speedup build of C# extension and protoc artifacts (#28225)

tools/run_tests/artifacts/build_artifact_csharp.bat

@@ -35,7 +35,7 @@ if "%ARCHITECTURE%" == "x64" (
 )
 
 call "%VS140COMNTOOLS%..\..\VC\vcvarsall.bat" %ARCHITECTURE%
-cmake -G Ninja -DCMAKE_C_COMPILER="%MSVC_COMPILER%" -DCMAKE_CXX_COMPILER="%MSVC_COMPILER%" -DCMAKE_BUILD_TYPE=RelWithDebInfo -DgRPC_BUILD_TESTS=OFF -DgRPC_MSVC_STATIC_RUNTIME=ON -DgRPC_XDS_USER_AGENT_IS_CSHARP=ON ../../.. || goto :error
+cmake -G Ninja -DCMAKE_C_COMPILER="%MSVC_COMPILER%" -DCMAKE_CXX_COMPILER="%MSVC_COMPILER%" -DCMAKE_BUILD_TYPE=RelWithDebInfo -DgRPC_BUILD_TESTS=OFF -DgRPC_MSVC_STATIC_RUNTIME=ON -DgRPC_XDS_USER_AGENT_IS_CSHARP=ON -DgRPC_BUILD_MSVC_MP_COUNT=4 ../../.. || goto :error
 cmake --build . --target grpc_csharp_ext
 cd ..\..\..
 

tools/run_tests/artifacts/build_artifact_protoc.bat

@@ -22,7 +22,7 @@ cd cmake
 mkdir build
 cd build
 
-cmake -G "%generator%" -DgRPC_BUILD_TESTS=OFF -DgRPC_MSVC_STATIC_RUNTIME=ON ../.. || goto :error
+cmake -G "%generator%" -DgRPC_BUILD_TESTS=OFF -DgRPC_MSVC_STATIC_RUNTIME=ON -DgRPC_BUILD_MSVC_MP_COUNT=4 ../.. || goto :error
 cmake --build . --target protoc --config Release || goto :error
 cmake --build . --target plugins --config Release || goto :error
 cd ..\..

tools/run_tests/helper_scripts/pre_build_csharp.bat

@@ -28,7 +28,7 @@ cd build
 mkdir %ARCHITECTURE%
 cd %ARCHITECTURE%
 
-cmake -G "Visual Studio 14 2015" -A %ARCHITECTURE% -DgRPC_BUILD_TESTS=OFF -DgRPC_MSVC_STATIC_RUNTIME=ON  -DgRPC_XDS_USER_AGENT_IS_CSHARP=ON ../../.. || goto :error
+cmake -G "Visual Studio 14 2015" -A %ARCHITECTURE% -DgRPC_BUILD_TESTS=OFF -DgRPC_MSVC_STATIC_RUNTIME=ON  -DgRPC_XDS_USER_AGENT_IS_CSHARP=ON -DgRPC_BUILD_MSVC_MP_COUNT=4 ../../.. || goto :error
 
 cd ..\..\..\src\csharp