[Security] PostgreSQL backup leaks password in command-line

[Natureshadow]

The PostgreSQL dumping code produces a database URI that is then passed verbatim on the pg_dump command-line.

Command lines are considered public, because normally, everyone with shell access to a system can read them (by calling ps or looking inside /proc).

All utilities using libpq can be configured using environment variables for this exact reason, which I will propose in a merge request.

As of Django 3.2, the DatabaseClient class will have a settings_to_cmd_args_env method that does exactly this, but we cannot yet use it for backwards compatibility.

[Archmonger]

Resolved in #560