System.Environment contains cross-platform operations to query environment variables, not to modify them. System.Posix.Env (package unix) provides operations to set environment variables on Linux (and probably other unices). This ticket requests adding cross-platform operations to set env vars to the module System.Environment.

As a workaround i copied the implementation of System.Posix.Env.putEnv into a new module. That seems to work on windows and linux (for me. Maybe this is due to using mingw/msys, though).