enable local-executor remote execution in buildbox-casd

[abderrahim]

buildbox-casd supports being a remote execution server, either as a proxy (which we take advantage of since #1926) or as a standalone server. It can also use hybrid local / remote execution and fallback (use local execution if remote returns an error).

Different features were introduced at different points, so we should be careful when choosing what to require and what to support optionally.

This would be useful in combination with #1985 to point a remote execution aware client (bazel or RECC) to use it instead of building in the sandbox. It can also be useful to configure buildbox-casd for hybrid execution.

[juergbi]

While the local executor was already enabled in #2014, it's currently still limited by not supporting a remote action cache with local execution. This limitation is being addressed by #2031.

[abderrahim]

Closing as the above PRs were merged already.