Support BuildStream as execution client

Context

BuildStream will soon gain a remote execution client (BuildStream/buildstream!626 (merged)).

Of course it is expected to play well with BuildGrid.

Task Description

BuildStream remote execution jobs target the buildbox bot, so the task here would consist in ensuring that the bot behave properly.

Acceptance Criteria

First stage, simple builds:

• Build BuildStream's autotools example project.

Second stage, complex builds:

• Build (parts of) the freedesktop-sdk?
• Build any other interesting project?