appveyor: fix issue self-upgradig pip to v10 (#663)

Installing with --user will leave the old pip.exe script in the $PATH,
but running this will fail because pip 10 moved 'main' to internal
modules.

https://github.com/pypa/pip/issues/5240#issuecomment-382989420
diff --git a/scripts/appveyor.yml b/scripts/appveyor.yml
index c7881ad..8f48e57 100644
--- a/scripts/appveyor.yml
+++ b/scripts/appveyor.yml
@@ -31,7 +31,7 @@
 install:

 - IF "%BUILD_SYSTEM%"=="Python" (

     SET "PATH=%PYTHON%;%PYTHON%\Scripts;%PATH%" &&

-    pip install --disable-pip-version-check --user --upgrade pip &&

+    python -m pip install --upgrade pip &&

     pip install --upgrade setuptools

   )

 - IF "%BUILD_SYSTEM%"=="make" (