[windows] split python installation off into its own block

windows/base/Dockerfile

@@ -20,11 +20,7 @@ RUN [Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tl
         curl \
         7zip \
         wixtoolset; \
-    Must-Run choco install -y --allow-multiple-versions python --version 3.8.7; \
-    Must-Run choco install -y --allow-multiple-versions python --version 3.7.9; \
     $env:PATH = "\"$([Environment]::GetEnvironmentVariable('PATH', [EnvironmentVariableTarget]::User));$([Environment]::GetEnvironmentVariable('PATH', [EnvironmentVariableTarget]::Machine))\""; \
-    Must-Run /Python38/python -m pip --no-cache-dir install --no-warn-script-location wheel; \
-    Must-Run /Python37/python -m pip --no-cache-dir install --no-warn-script-location wheel; \
     Must-Run choco install -y visualstudio2017buildtools; \
     Must-Run choco install -y visualstudio2017-workload-vctools; \
     Start-Process "\"C:/Program Files (x86)/Microsoft Visual Studio/Installer/vs_installer.exe\"" -NoNewWindow -Wait -ArgumentList 'modify --installPath \"C:/Program Files (x86)/Microsoft Visual Studio/2017/BuildTools\" --quiet --norestart --remove Microsoft.VisualStudio.Component.VC.CMake.Project --add Microsoft.VisualStudio.Component.VC.Tools.14.16 --add Microsoft.VisualStudio.Component.VC.ATL'
@@ -89,3 +85,9 @@ RUN cd $env:VCPKG_DIR; \
     Remove-Item -Force -Recurse ./downloads; \
     Remove-Item -Force -Recurse ./buildtrees; \
     Remove-Item -Force -Recurse ./packages;
+
+RUN Must-Run choco install -y --allow-multiple-versions python --version 3.8.7; \
+    Must-Run choco install -y --allow-multiple-versions python --version 3.7.9; \
+    $env:PATH = "\"$([Environment]::GetEnvironmentVariable('PATH', [EnvironmentVariableTarget]::User));$([Environment]::GetEnvironmentVariable('PATH', [EnvironmentVariableTarget]::Machine))\""; \
+    Must-Run /Python38/python -m pip --no-cache-dir install --no-warn-script-location wheel; \
+    Must-Run /Python37/python -m pip --no-cache-dir install --no-warn-script-location wheel