Checking for key expiry on dak new-security-install

[Moritz Mühlenhoff <jmm@inutil.org>]

Dear FTP masters,
The wanna-build team is re-signing the builds, which had been rejected
due to the expired buildd keys.

ndependently of that I would suggest to change dak to _not_ do 
additional key expiry checks during dak new-security-install. It's 
of course needed to check the expiry during upload, but doing a 
second check on install sounds like a ticking time bomb with key 
change intervals of 90 days.

Just think of security updates with long embargos or packages waiting
in proposed-updates because of missin builds etc.

Cheers,
        Moritz