.github: Switch to v3 of actions/checkout.

GitHub is emitting a warning that v2 is deprecated due to using
Node.js 12.

Reported by: GitHub
Reviewed by: arichardson, emaste
Differential Revision: https://reviews.freebsd.org/D37769

(cherry picked from commit 54f3a781a2ff980034dfe13ff6323c96fc587f1b)