bin/install_megadrivers.py: Fix regression for set DESTDIR

The previous patch tried to address a bug when DESTDIR is '', however,
it introduces a bug when DESTDIR is not '', and fakeroot is used. This
patch does fix that, and has been tested with the arch pkg-build to
ensure it isn't regressed.

Fixes: 093a1ade4e24b7dd701a093d30a71efd669fe9c8
       ("bin/install_megadrivers.py: Correctly handle DESTDIR=''")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110221


Reviewed-by: Eric Engestrom <eric@engestrom.ch>
(cherry picked from commit ed96038e)