Test streaming of standard outputs

Before raising this MR, consider whether the following are required, and complete if so:

• Unit tests
• Metrics
• Documentation update(s)

Not applicable.

Description

This adds a step to test the streaming of standard outputs between the worker and BuildGrid using bgd read as a third-party reader.

Changes proposed in this merge request:

• Compile buildbox-worker with the necessary flags to enable its --launch-logstream-command testing option
• Make the worker invoke a helper script that launches bgd read and dumps the output to a file
• Add a new run_recc_and_check_logstream step that simulates a compiler error
• Check that the output observed by bgd read matches the output that recc received after downloading the output blobs from the CAS.

(Depends on the changes proposed in buildbox-worker!90 (merged) and requires removing a commit from the history before merging.)

Issues addressed

Closes #9 (closed)