zulip: upgrade-postgres script fails on Ubuntu 18.04 because of unavailable postgresql-12-pgroonga

The upgrade-postgres script fails on Ubuntu 18.04, because the bionic dist of the Groonga repo has no package postgresql-12-pgroonga, only postgresql-10-pgroonga.

It fails with:

Error: Execution of '/usr/bin/apt-get -q -y -o DPkg::Options::=--force-confold install postgresql-12-pgroonga' returned 100: Reading package lists...
Building dependency tree...
Reading state information...
E: Unable to locate package postgresql-12-pgroonga
Error: /Stage[main]/Zulip::Postgres_appdb_base/Package[postgresql-12-pgroonga]/ensure: change from 'purged' to 'present' failed: Execution of '/usr/bin/apt-get -q -y -o DPkg::Options::=--force-confold install postgresql-12-pgroonga' returned 100: Reading package lists...
Building dependency tree...
Reading state information...
E: Unable to locate package postgresql-12-pgroonga
Notice: /Stage[main]/Zulip::Postgres_appdb_base/File[/usr/share/postgresql/12/pgroonga_setup.sql]: Dependency Package[postgresql-12-pgroonga] has failures: true
Warning: /Stage[main]/Zulip::Postgres_appdb_base/File[/usr/share/postgresql/12/pgroonga_setup.sql]: Skipping because of failed dependencies
Notice: /Stage[main]/Zulip::Postgres_appdb_base/Exec[create_pgroonga_extension]: Dependency Package[postgresql-12-pgroonga] has failures: true
Warning: /Stage[main]/Zulip::Postgres_appdb_base/Exec[create_pgroonga_extension]: Skipping because of failed dependencies
Notice: Applied catalog in 10.23 seconds

I fixed this by manually downloading the package from the pool and installing it.