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.

pypa/pip#5240 (comment)

scripts/appveyor.yml

@@ -31,7 +31,7 @@ environment:
 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" (