[CI] Add yarn cleaninstall via nix develop to Buildkite

Summary: Run yarn cleaninstall within nix environment and make sure it completes successfully.

Test Plan:
Worked here:
https://buildkite.com/comm/nix-develop-darwin/builds/5#01824144-95d8-4952-b9c4-5500577c3ccb

Reviewers: jon, varun, abosh, tomek, jacek

Reviewed By: jon, varun, abosh

Subscribers: ashoat, adrian

Differential Revision: https://phab.comm.dev/D4830