PSA for Linux users: work-around for Linux-nightly extension manager error

Here is a temporary work-around for the following bug, which prevents using the extension manager in nightly builds on Linux:

in the expanded Slicer directory, copy the contents of resources/ to libexec. Then restart Slicer.

root@kali:~/Downloads/Slicer-4.9.0-2018-05-02-linux-amd64# cp resources/* libexec/
root@kali:~/Downloads/Slicer-4.9.0-2018-05-02-linux-amd64# ./Slicer

This should be permanently addressed (updating linux builds to 5.11) in the next nightly build (fingers crossed).