[FIX] digest: force recompute of all kpis each time-based run

Before this commit, the computation of kpi for last week, last 30 days and
previous period comparison was wrong due to cache. Indeed as first value
was in cache, other values were taken directly from cache itself instead
of recomputing each value based on start_date and end_date of the computation
timeframe.

As digest fields are computed fields used a bit off-side, let us manually
invalidate the cache before computing a kpi so that it correctly computes
the timeframe values.

Task ID 1883428

closes odoo/odoo#40834

Closes: #40834
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>