New mirror.

builder/stage0/00-configure-apt/files/sources.list

@@ -1,3 +1,3 @@
-deb http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi
+deb http://mirror.umd.edu/raspbian/raspbian buster main contrib non-free rpi
 # Uncomment line below then 'apt-get update' to enable 'apt-get source'
 #deb-src http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi