Any package `X` used as `depends_on("x", type="build")` will have
`X.setup_run_environment(env)` called, because it has to be able to
"run" in the build environment.
So there is no point in calling `setup_run_environment` from
`setup_dependent_build_environment`.
Also it's redundant to call `setup_run_environment` in
`setup_dependent_run_environment`, cause (a) the latter is called _for
every parent edge_ instead of once per node, and (b) it's only called
after `setup_run_environment` is called anyways. Better to call
`setup_run_environment` once and only once.
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