I believe you have three options with advantages/disadvantages as mentioned. Describing the options in detail as per my understanding because similar questions of summing rows are often there.
1) Add a Virtual Column called [resultat] : Easy to implement but expensive for app sync time and the sum value will not be stored in the spreadsheet.
Expression for [resultat]
SUM ( SELECT(Table Name[Colonne heure journée/], [Colonne chantier]=[_THISROW].[Colonne chantier]))
Please replace the table name with your table name.
2) Add a real column and spreadsheet formula: Will store the results in the spreadsheet but the [resultat] will update while adding a new record only after the latest record is saved. The sync time will be somewhat expensive. The spreadsheet formula for [resultat] column each row can be as below
Please note the middle part of the formula (A2) will change to A3, A4 … for each row.
3) Use reference actions to update the real column. The implementation may be a bit cumbersome. The sync time may be consumed while adding record but it could be in the background. This option could be used if handful of records and not large number of records are to be updated, because each record will be updated by an action.