Fix check failure in test_process_instruction_compute_budget

[yhchiang-sol]

Problem

Looks like the master is failing the follow check in test_process_instruction_compute_budget:

error[E0560]: struct `solana_sdk::transaction_context::InstructionAccount` has no field named `index`
--
  | --> program-runtime/src/invoke_context.rs:1677:17
  | \|
  | 1677 \|                 index: account_index,
  | \|                 ^^^^^ `solana_sdk::transaction_context::InstructionAccount` does not have this field
  | \|
  | = note: available fields are: `index_in_transaction`, `index_in_caller`, `is_signer`, `is_writable`

Summary of Changes

This PR fixes the check failure by assigning account_index to the right field.

[carllin]

Thanks for the fix!

[codecov]

Codecov Report

Merging #22202 (c80c851) into master (e1a0660) will decrease coverage by 0.1%.
The diff coverage is 61.1%.

@@            Coverage Diff            @@
##           master   #22202     +/-   ##
=========================================
- Coverage    81.2%    81.1%   -0.2%     
=========================================
  Files         520      523      +3     
  Lines      145949   146609    +660     
=========================================
+ Hits       118534   118910    +376     
- Misses      27415    27699    +284