Environmental variables exported in `command` don't propagate

We're trying to write a [plugin](https://github.com/CampusJob/kubernetes-logs-buildkite-plugin) configurable by the user in the `command` phase of the job, having the user set environmental variables then to be used by the `post-command` plugin hook.

We have found that when environmental variables are set in the `command` phase by the user, even using `export`, the variables fail to propagate to the `post-command` of the plugin.