Publish build output via Buildkite (and GCS)

[fweikert]

By default, all build output should be available via Buildkite in the "Artifacts" tab. Moreover, users should be able to tell Buildkite to upload output to GCS, too.

[philwo]

@fweikert What do you mean with "build output"? The build log? Or the entire bazel-out folder?

[fweikert]

IIRC users asked for bazel-out

[mostynb]

This feature would be useful for me, at least (working on bazel-remote). I can see that bazelci.py has a --save_but flag, but I'm not sure where to add it.