[ChangeLog][Build System] Tools that are called by the build system and
are unlikely to be called by the user are now installed to the libexec
directory.
This is a step towards easier co-installability of different Qt
versions.
Pick-to: 6.1
Task-number: QTBUG-88791
Change-Id: Id19575b5ba27795f7715e4ea6a09391b26dd4942
Reviewed-by: Kai Koehne <kai.koehne@qt.io>