wireless: T4287: use Debian postinst over preinst when using update-alternatives

This fixes an error during ISO assembly:

update-alternatives: error: no alternatives for regulatory.db
dpkg: error processing archive /tmp/apt-dpkg-install-PJplR3/00-vyos-1x_1.5dev0-1880-gecaa44498_amd64.deb (--unpack):
new vyos-1x package pre-installation script subprocess returned error exit status 2
(cherry picked from commit a414190447c32be0775a077cde13cef0cf2b8c54)