Make /mit/debathena/apt/db world-readable

[vasilvv]

I see no reason for this to be read-restricted to buildserver and debathena-root, and I find the fact I can't read it with my null instance fairly annoying in practice, because I can't automatically resnapshot my local APT copy).

I intend to fix this unless someone tells me there's a good reason for this.

[achernya]

No objections seen in two weeks; fs sa'd system:anyuser rl.