[CI] Use System Qt5.12 on windows Buildbot
Proof tested on-site.
Context: Qt5.12.9 is required for upcomming bug fixes and by the future PV5.9. All platforms will use this SYSTEM Qt5.12.9. Post-PV5.9, this change will stay depending on various factors:
- Buildsbots toolchains must be upgraded on windows to be able to build Qt5.12.9 itself
- Ubuntu < 20.04 cannot built Qt5.12.9 with the default supplied GCC