Small follow-up on #2128

[paul0403]

Context:
Recently plxpr pipeline's control flow handling of consts for subscope body jaxprs were updated, but one place was missed in the update. #2128

Description of the Change:
handle_for_loop correctly uses length of new_consts.

Benefits:
Things are correct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.39%. Comparing base (6c68848) to head (c32b845).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2133      +/-   ##
==========================================
- Coverage   97.42%   97.39%   -0.03%     
==========================================
  Files          91       91              
  Lines       10610    10610              
  Branches      999      999              
==========================================
- Hits        10337    10334       -3     
- Misses        216      218       +2     
- Partials       57       58       +1     

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