Build leaks host kernel version into artifact

Since QTBUG-129996 was fixed with this implementation, the build includes the CMAKE_SYSTEM_VERSION in the json metadata, which by default is set to the host system version.

This makes the build not reproducible: the Linux Kernel version of the build system leaks into the artifact. It seems overly specific as well: AFAICS Qt does not call Linux Kernel APIs directly, but relies on system libraries - so the built Qt artifacts should (and do) run fine on various Linux Kernel versions.

Presumably we could work around this by setting CMAKE_SYSTEM_VERSION to a dummy value, but it begs the question: wouldn't it be better to leave out this value entirely?