Add --distro flag to PPA release script

Summary:
This makes it easier to:

• Test fixes only relevant to one distro.
• Re-upload a fix only to relevant distros, rather than bumping the version number for unaffected ones.

Depends on D6927 for help text change.

Test Plan:

./debian-packages.sh --help
./debian-packages.sh --dry-run jasonbcox
./debian-packages.sh --dry-run -D focal jasonbcox

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6928