Using key fingerprint instead of name

Apt::Key uppercases the key name, and then does a case-sensitive grep
for that name on the output of apt-key list. This causes the check to
fail on the name (as it contains lowercase characters), causing the key
to be re-added each time. Using the key fingerprint instead avoids this
issue, and reduce the risk of potential naming issues in case the key is
changed in the future.