Fix Debian package script in case signer has multiple user IDs

Summary:
The script breaks if the signer has multiple, ultimately-trusted user IDs.
I recently added another email to my GPG key, so this was broken for me.

Test Plan:
Used this script to upload to my staging PPA: https://launchpad.net/~jasonbcox/+archive/ubuntu/bitcoin-abc/+packages
./debian-packages.sh -p jasonbcox-bitcoin-abc jasonbcox

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6329