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: 093a1ade ("bin/install_megadrivers.py: Correctly handle DESTDIR=''")