Currently, payslip computation for Pakistan localization calculates incorrect tax when the yearly cost exceeds 2,200,000.

### **Steps to Reproduce:**
1) Install `l10n_pk_hr_payroll_account`,`hr_contract_salary` with demo data. 
2) Switch to PK company.
3) Create an employee and a running contract with yearly cost 2,200,001 
4) Create a payslip and compute it from the Salary Computation tab.

### **Observed Behavior:**
'Tax Bracket Yearly' is computed as `122000.24`

### **Expected Behavior:**
'Tax Bracket Yearly' should be `116000.24`

### **Root Cause:**
since [commit](https://github.com/odoo/enterprise/pull/98345/commits/62f5e216518d83317618d7dbc0be4db92d1881a3), the tax computation relies on  [_l10n_pk_get_tax](https://github.com/odoo/enterprise/blob/a9656336583a4d5c5b12d4d6120ec62ba1cf9151/l10n_pk_hr_payroll/models/hr_payslip.py#L9-L20) , In this method, the `result` is incorrectly accumulated with `fix` when iterating through brackets, leading to an inflated tax value.

[official pakistan document](https://download1.fbr.gov.pk/Docs/20258181281745641WHT-RateCard.pdf)

### **Fix:**
Use `result = fix` instead of `result += fix` so that the cumulative tax is correctly reset at each bracket.

**opw-5979265**

Forward-Port-Of: odoo/enterprise#110939