nsis: Switch early to 64-bit registry for 64-bit installations

This fixes the problem that the installer always asked for the language.
The MUI_LANG* code saves the chosen installer language at the end of the
installation to the 64-bit registry (for 64-bit installations). Thus
it makes no sense to force MUI_LANGDLL_DISPLAY to read it from the
32-bit registry. Instead we switch to the 64-bit registry at the start
of .onInit (as done at the start of un.onInit) for 64-bit installations.

Incidentally, this also fixes the problem that 32-bit installations
would have used the 64-bit registry (until the first switch in one of
the other .nsi files).