arrow

GH-44991: [CI][Python] Fix and modernize AppVeyor build (#44999)

1. Use Mambaforge instead of outdated Miniconda install
2. Install PyArrow using pip, not setup.py

Also fixes GH-44997.

* GitHub Issue: #44991

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <pitrou@free.fr>