If it adds to the environment then it ultimately creates the same
kind of problem it was intended to solve--a need to reproduce
the calls to `subprocess` in the code outside. It's not that hard
to merge two dicts, plus much of the time enough you can get by with
having just `PATH` and `AMARANTH_ENV_*` (if even that).
If an override is wanted it can be done easily enough with:
.execute_local(env={**os.environ, "VAR": "VALUE"})
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