pkg: use https by default

Switch the repository to use https by default, base is providing a CA
root bundle suitable to validate the certificates used by the project.
This can now be activated without requiring another packages to be installed

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D40473

(cherry picked from commit d557a86c879a8515d59e8380b083b2265e9a3547)