This adds a line to `set --show`s output like
```
$PATH: originally inherited as |/home/alfa/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl:/var/lib/flatpak/exports/bin|
```
to help with debugging.
Note that this means keeping an additional copy of the original
environment around. At most this would be one ARG_MAX's worth, which
is about 2M.

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