Making the SD activity LED work on the Raspberry Pi 3 AArch64 Debian Buster image

I tried installing Gunnar Wolf's Debian Buster Image on my Raspberry Pi 3. I found that the SD card activity LED didn't work by default. This can easily be fixed (until the next reboot) with:

echo mmc0 > /sys/class/leds/ACT/trigger

Attempting to set the trigger to the more generic disk-activity didn't seem to work.