Making the apt::key regsubst get less chars

Sometimes keys don't have 8 chars (like 'Google'), but I think 3 the minimum.
Making the regex get the last 3 to 8 chars.
Tested with Rubular to assure the result is the same as the old regex.

One Issue is if the developer put a really generic key, like 'red'. It would match RedHat and Redmine. I don't think this is a great issue anyway.