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

Needed for commit 9c8d2b7be3 (#21115).

Signed-off-by: Anders Kaseorg <anders@zulip.com>
(cherry picked from commit 7213116dd3)

tools/lib/provision.py

@@ -278,6 +278,7 @@ def install_apt_deps(deps_to_install: List[str]) -> None:
             "apt-get",
             "-y",
             "install",
+            "--allow-downgrades",
             "--no-install-recommends",
             *deps_to_install,
         ]