[GH Actions] Include brew install protobuf step in iOS build workflow

Summary:
The GH Actions iOS build workflow was failing because the prost_build crate requires protoc.

Since the GH Actions macOS runners are x86 machines, brew will install protoc in usr/local/bin so we shouldn't need to do any symlinking in order for protoc to be picked up by xcodebuild.

Test Plan: See what happens (usually can test changes to workflows on a fork, but it's particularly annoying for iOS builds)

Reviewers: ashoat, tomek, marcin, abosh, atul

Reviewed By: ashoat

Subscribers: varun, jon, abosh

Differential Revision: https://phab.comm.dev/D5133