When onActivation.cleanup == "check", a separate `brew bundle cleanup`
command is run in the system.checks phase before the actual activation.
This `brew bundle cleanup` invocation should use the same `extraEnv`
as the main `brew bundle [install]` command, for consistency.
For example, when $XDG_CONFIG_HOME is set in `extraEnv`, it should also
be picked up by `brew bundle cleanup` in the system.checks phase.
Bryan Lai