Azure CI: Be explicit about pytest cache directory

Azure CI: Be explicit about pytest cache directory

The default pytest cache directory is in a read-only directory in Azure,
which results in a warning on the build page. Use the pytest command
line option to set the cache dir to somewhere writable.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>