Portals are global so we can just link them globally.
There might, in theory, be some unexpected system-path contamination
(e.g. when a portal package installs its executables to `/bin`)
but I think the risk is relatively minor compared to the added complexity.
While at it, let’s point the environment variable to system-path.
That will allow changes to installed portals to apply without having to re-log in.
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