[CI] Update the arcanist mirror

Summary:
we.phorge disabled anonymous cloning:
https://we.phorge.it/phame/post/view/8/anonymous_cloning_disabled_on_phorge.it/, let's use the github mirror instead

Test Plan: Run any CI build.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18299