Revert ninja installation to /windows/system32

windows/base/Dockerfile

@@ -14,8 +14,9 @@ RUN [Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tl
         7zip \
         wixtoolset; \
     (New-Object Net.WebClient).DownloadFile('https://github.com/Kitware/ninja/releases/download/v1.8.2.g972a7.kitware.dyndep-1/ninja-1.8.2.g972a7.kitware.dyndep-1_i686-pc-windows-msvc.zip', 'C:/ninja.zip'); \
-    7z e C:/ninja.zip -oC:/Windows/System32 -y; \
+    7z e C:/ninja.zip -oC:/ninja -y; \
     Remove-Item -Force C:/ninja.zip; \
+    [Environment]::SetEnvironmentVariable('PATH', "$([Environment]::GetEnvironmentVariable('PATH', [EnvironmentVariableTarget]::Machine))" + ';C:/ninja', [EnvironmentVariableTarget]::Machine); \
     choco install -y python --version 3.7.1; \
     $env:PATH = "\"$([Environment]::GetEnvironmentVariable('PATH', [EnvironmentVariableTarget]::User));$([Environment]::GetEnvironmentVariable('PATH', [EnvironmentVariableTarget]::Machine))\""; \
     python -m pip --no-cache-dir install --no-warn-script-location wheel