provision: Use apt-get --allow-downgrades.
provision: Use apt-get --allow-downgrades.

Needed for commit 9c8d2b7be39314b41f04a1e1356942afe21ab223 (#21115).

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

zulip/zulip,rht/zulip,kou/zulip,kou/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,andersk/zulip,kou/zulip,kou/zulip,kou/zulip,rht/zulip,kou/zulip,zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip,kou/zulip,zulip/zulip,rht/zulip,zulip/zulip,zulip/zulip,zulip/zulip