Update FindQt5.cmake to fix ATLINFR-2967
This MR pushes a change by @akraszna to fix ATLINFR-2967.
With the change in this MR, the env var QTLIB is set even if already defined on the machine. This fixes the build on standard Lxplus-like machines. [We didn't see errors on ATLAS build nodes, because there is no system Qt5 installed on them]