Cirrus-CI: fix git usage by build user

The git checkout it owned by root, but builds are run as "user". git
refuses to operate in such an environment unless the directory is
trusted so make "user" trust it.

Fixes CI after 99b8c0c35b0fcc633649209621243d678a13542a.

Sponsored by: DARPA

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D42903

(cherry picked from commit 663e4fa38fe835056b24058acc6e43e4a15a84c5)