Allow --user-area to point to a nightly build slot installation
The special behaviour introduced with !125 (merged) is not working when the nightly build slot is installed in a directory different from <slot>/<build_id>
(as it is the case for LHCbPR tests).
This MR tweaks the behaviour of --user-area
to match !125 (merged) when it points to a nightly build slot installation.