Added `build.id` variable and other improvements / fixes

Summary: This adds a `build.id` variable, cleans up the naming convention of other variables and also fixes an issue in the remote command to read the buffers after the command finishes.

Test Plan: Ran a build with `/bin/echo ${build.id}` and saw the build ID come through.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7540