build: limit parallel processes

Since the precompile headers (PCH) take up so much RAM, we can't use all of the CPU cores without hitting out of memory errors.

Author: dlech

build.ps1

@@ -15,6 +15,6 @@ $buildPath = "$repoRootPath\_build\$env:VSCMD_ARG_TGT_ARCH-$buildType"
 $packagePath = "$sourcePath\pywinrt"
 
 cmake -S $sourcePath "-B$buildPath" -GNinja "-DCMAKE_BUILD_TYPE=$buildType" -DCMAKE_C_COMPILER=cl -DCMAKE_CXX_COMPILER=cl "-DPYTHON_VERSION_STRING=$pythonVersion"
-cmake --build $buildPath -- -v 
+cmake --build $buildPath -- -v -j 4
 
 Copy-Item "$buildPath\*.pyd" "$packagePath\winsdk\"