shadowsocks: Fails to run in buster

With the changes in FreedomBox v20.4, Shadowsocks is failing to run in buster, because /var/lib/private/shadowsocks does not exist. This is likely because the DynamicUser setting was added in 3.3.3+ds-1, while buster has 3.2.5+ds-1.

A quick solution may be to use the version in backports, 3.3.4+ds-1~bpo10+1.