Standalone qmlls binary: package from installation folder

From the comments on https://codereview.qt-project.org/c/qt/qt5/+/580387?tab=comments that still needs to be resolved:

Note for the future.
It's probably ok if you do it like this atm, but ideally it would be better to build and install all of qt, and package qmlls from the installed location, rather than from the build dir.
For static builds it's less relevant because there are fewer dynamic dependencies, but still it would likely be safer if there are system dynamic lib deps, especially on linux.

Another point: CMake also does its rpath translations on installation time.
While this isn't affecting us at the moment it could bite us in the future.
But for the first iteration we can go with the current approach.

So: build + install all of Qt in the workflows script, and package qmlls from the installation folder.