Add BUILDKITE_ORGANIZATION_ID to the list of supported environment variables

**Is your feature request related to a problem? Please describe.**
It would be useful to have the organization ID (e.g. as used in calculating artifact upload URLs) exposed as an environment variable to hooks (e.g. for writing plugins that want to manually calculate artifact URLs). At the moment, it is nontrivial to determine the organization ID from within, for instance, a plugin hook.

**Describe the solution you'd like**
I would like a `BUILDKITE_ORGANIZATION_ID` environment variable exposed by the buildkite agent, similarly to `BUILDKITE_PIPELINE_ID`, `BUILDKITE_JOB_ID`, etc...

**Describe alternatives you've considered**
I have considered requiring the user to provide the organization ID as a parameter to my plugin, or even a token to allow the plugin hook to look up the organization ID from the web API, but neither are particularly useful.