I see that 90-digispark.rules for Linux sets MODE to 0666, giving write permission to everyone. I'm setting GROUP instead:
SUBSYSTEMS=="usb", ATTRS{idVendor}=="16c0", ATTRS{idProduct}=="05df", GROUP="adm"
and put it into /etc/udev/rules.d (as root, of course). I'm in the 'adm' group already on this single-user Ubuntu machine, but check that you are on yours using groups.