[FIX] spreadsheet_dashboard_account: count individual invoices

In the Invoicing dashboard, the "Average Invoice" scorecoard displays
the wrong amount of invoices. It displays the number of
"account.invoice.report" lines.

This commit fixes the issue by displaying the count_unique measure
of "move_id"

task 3180524

closes odoo/odoo#120253

Signed-off-by: Rémi Rahir (rar) <rar@odoo.com>