[CI] propagate buildkite env vars to the docker container

Summary: This lets us check if we're in a buildkite workflow before deciding whether or not to install deps. We were already manually passing in the BUILDKITE env var in the android CI, but this approach is cleaner I think.

Test Plan: CI should pass

Reviewers: jon, atul

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D6261