[GH Actions] Set DEVELOPER_DIR to Xcode 15.0

Summary: Context: https://linear.app/comm/issue/ENG-5313/ios-build-is-broken-following-landing-of-d9470

Test Plan: Land and revert if it fails

Reviewers: ashoat, tomek, marcin, bartek, varun

Reviewed By: ashoat

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9539