pureos: Temporarily add amber-proposed-upates

We only need this until librem5-devkit-base >= 0.0.3 migrated but without
that we don't get a bootable kernel.

conf/dists/amber-phone-staging

@@ -5,6 +5,8 @@ deb https://repo.pureos.net/pureos amber-security main
 deb https://repo.pureos.net/pureos amber-phone main
 deb https://repo.pureos.net/pureos amber-phone-staging main
 
+deb https://repo.pureos.net/pureos amber-proposed-updates main
+
 deb http://repo.pureos.net/pureos-debug/ amber-debug main
 deb http://repo.pureos.net/pureos-debug/ amber-phone-staging-debug main
 "