bpo-30450: Fix logic for retrying nuget.exe download (#2744)

Fix logic for retrying nuget.exe download with Python.
Add support for HOST_PYTHON variable.
Clear internal environment variables used in find_python.bat
Use HOST_PYTHON as the actual Python if it is recent enough.
Adds HOST_PYTHON variable to AppVeyor configuration

.github/appveyor.yml

@@ -9,6 +9,8 @@ build_script:
 - cmd: PCbuild\build.bat -e
 test_script:
 - cmd: PCbuild\rt.bat -q -uall -u-cpu -rwW --slowest --timeout=1200 --fail-env-changed -j0
+environment:
+  HOST_PYTHON: C:\Python36\python.exe
 
 # Only trigger AppVeyor if actual code or its configuration changes
 only_commits: