[racket] Environment variables in paths

I can't seem to find a path function that deals with environment variables.
Is there some variant of cleanse-path that resolves environment variables
first?

Concretely, I have an environment variable $WORKSPACE that tells where I'm
operating from. The result of (getenv "WORKSPACE") is the correct absolute
path, but (build-path "$WORKSPACE" ...) doesn't resolve the variable, so my
filesystem commands are failing.

Thanks.
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