nixos-18.03/pgbackup: Fix bug in postgresql-backup module that causes to fail

[markuskowa]

Motivation for this change

The postgresqlBackup module fails to backup the database.
User root is ignored and pg_dump does not create a backup (at least in newer postgres version?).
See #41388

This is a hotfix for 18.03. See #42133 for a more thorough
refactoring of the postgresqlBackup module.

Things done

Run pg_dump as postgres super user instead of root. Otherwise

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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