appveyor: Add a flag to work with the new version of setuptools (v50.0)

From the official documentation: "Once again, Setuptools overrides the
stdlib distutils on import. For environments or invocations where this
behavior is undesirable, users are provided with a temporary escape hatch.
 If the environment variable SETUPTOOLS_USE_DISTUTILS is set to stdlib,
Setuptools will fall back to the legacy behavior. Use of this escape
hatch is discouraged, but it is provided to ease the transition while
proper fixes for edge cases can be addressed."

Signed-off-by: Alexandra Trifan <Alexandra.Trifan@analog.com>

CI/appveyor/build_appveyor_msvc.sh

@@ -32,7 +32,7 @@ __build_libm2k() {
 	cmake --build . --config Release
 
 	"$PY_PATH/python.exe" -m pip install --user --upgrade setuptools wheel
-	"$PY_PATH/python.exe" setup.py bdist_wininst
+	SETUPTOOLS_USE_DISTUTILS=stdlib "$PY_PATH/python.exe" setup.py bdist_wininst
 	"$PY_PATH/python.exe" setup.py sdist bdist_wheel --plat-name "$PLAT_NAME" --python-tag py"$PY_VERSION"
     cp dist/libm2k-*.exe "/c/projects/libm2k/build-$PLATFORM/dist/libm2k-py$PY_VERSION-$PLATFORM.exe"
 	cp dist/libm2k-*.whl "/c/projects/libm2k/build-$PLATFORM/dist/"