[CI] Add emojis to Buildkite pipelines

Summary:
Super quick change that adds a label to each Buildkite pipeline's YAML file.

You can see the emojis here: https://github.com/buildkite/emojis

Test Plan: Ran builds in Buildkite and saw emojis.

Reviewers: atul, palys-swm

Reviewed By: atul

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4486