Fix build with older Qt: Disable http2 for now due to Qt bug, add env var (upstream)

Fix build with older Qt: Disable http2 for now due to Qt bug, add env var (upstream)

Disable http2 for now due to Qt bug but allow enabling it via env var, see: https://github.com/owncloud/client/pull/7620
  and: https://github.com/nextcloud/desktop/pull/1806
Issue: https://github.com/nextcloud/desktop/issues/1503

Co-authored-by: XNG <Milokita@users.noreply.github.com>
Co-authored-by: Hannah von Reth <hannah.vonreth@owncloud.com>
Signed-off-by: Michael Schuster <michael@schuster.ms>