Cirrus-CI: add a manually triggered arm64 task

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31953

(cherry picked from commit 8cba2003e85b49a482ca623945209ca537374229)
(cherry picked from commit 3d7ec50621cac05f229651f966829248e6d70c8c)