[debian release] Fetch signer string from GPG rather than requiring the user to enter a perfectly formatted one

Summary:
This allows the releaser to type fewer characters with simpler formatting requirements.
See test plan for examples.

Test Plan:

./debian-packages.sh --dry-run jason
./debian-packages.sh --dry-run jasonbcox
./debian-packages.sh --dry-run jasonbcox@bitcoinabc.org
./debian-packages.sh --dry-run 3BB16D0
./debian-packages.sh --dry-run 1  # fails due to too many matches
./debian-packages.sh --dry-run blah  # fails due to no matches

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5552