Lock builds triggered with `!build`.

Summary:
- Prevents someone spamming `!build` over and over again in irc.
- Does **not** prevent someone doing `!build` and pushing a commit at the same time.
  - This should eventually be handled at a higher level.

Test Plan: compile

Reviewers: jcarter

Reviewed By: jcarter

Differential Revision: https://phabricator.edgesysdesign.com/D110