[ios] Add --repo-update to pod install command

Summary:
Sometimes pod install fails if it doesn't have up-to-date repo locally:

This just prevents that going forward.

Test Plan: Command runs fine. This is the only instance of pod install in the repo.

Reviewers: ashoat, ginsu, marcin, bartek

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7161