[GH Actions] Fix base64 command options for macOS 13

Summary:
Change in base64 command is causing iOS workflow to fail.

Luckily I've run into this before (https://github.com/atulsmadhugiri/blob/commit/abca30d8179a87cc764f65c51e90d0408646315c) and fix is straightforward.

Test Plan: Land and see what happens.

Reviewers: ashoat, tomek, bartek, varun

Reviewed By: varun

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9540