Allow --user-area to point to a nightly build slot installation

Marco Clemencicrequested to merge
allow-run-nightly-slot-from-dir into master

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.

Merge request reports