Add cancel_intermediate_builds to all buildkite pipelines

[keith]

A few pipelines have cancel_intermediate_builds set, which cancels running jobs when new commits are pushed to existing branches. I think we should enable this for all pipelines to reduce CI queuing, which is especially helpful for macOS CI. I would also recommend we set cancel_intermediate_builds_branch_filter = '!master' (or !main) to make sure this doesn't apply to the default branch

[keith]

This is especially useful since non-googlers(?) can't cancel builds manually: