install: never hit assert() when we can't figure out where to write configuration symlinks

Under specific circumstances it might happen that we can't figure out
where to place our symlinks, for example because we are supposed to
create them in the runtime directory but $XDG_RUNTIME_DIR is not set. In
this case, return -ENXIO instead of hitting an assert().

(Yeah, the error isn't very descriptive, but for now this should at
least be good enough to remove the assert() being hit.)

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