libaccounts-glib: Work around Python install for Qt6, too

When libaccounts-glib reaches its install phase, by default it attempts to install a Python module (a GObject introspection "overrides" file, basically a class wrapper) into the system's Python library dir. This will typically trigger a permissions prompt, blocking the build until it's dealt with. And if write access isn't obtained, the install fails.

This was previously addressed for Qt5 in 68596625, where Marius redirected the install location of the Python overrides file into a writable dir. This MR puts the same workaround in place for Qt6 builds.

It also updates the comments above the workaround in both files, noting that the workaround should hopefully be temporary. An upstream MR, to which the maintainers have signaled their basic approval, would make the Python module installation default-disabled, so that it would have to be explicitly requested. At that point, these workarounds can be removed.