fix: fetch currency in discount accounting SI

[GursheenK]

Problem
When using Discount Accounting, if a Sales Invoice is created using a different currency instead of the default currency for that company, the GL entries created consider the SI currency for the discount account but the company default currency for all other accounts. This leads to a mismatch in the total debit and total credit values and the SI cannot be submitted.

Solution
Use the base_discount_amount instead of the discount_amount in the GL entry for Discount Accounting.

Closes #36529

[codecov]

Codecov Report

Merging #36889 (112cfe6) into develop (cd74c6c) will increase coverage by 0.00%.
Report is 2 commits behind head on develop.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop   #36889   +/-   ##
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  Coverage    65.42%   65.42%           
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  Files          794      794           
  Lines        62194    62194           
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+ Hits         40690    40693    +3     
+ Misses       21504    21501    -3     

Files Changed	Coverage
erpnext/controllers/accounts_controller.py	ø