DM-40330: Fix counting of elements in loop

[timj]

Previously enumerate() was used in the loop as a proxy counter but this was removed mistakenly when ruff complained that the counter wasn't being used. Replace the logic with an explicit counter so people don't get confused again.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

Patch coverage: 66.66% and project coverage change: -0.15% ⚠️

Comparison is base (d1bbb55) 84.23% compared to head (0e05001) 84.09%.

❗ Current head 0e05001 differs from pull request most recent head 2712915. Consider uploading reports for the commit 2712915 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #365      +/-   ##
==========================================
- Coverage   84.23%   84.09%   -0.15%     
==========================================
  Files          77       77              
  Lines        9070     9071       +1     
  Branches     1739     1739              
==========================================
- Hits         7640     7628      -12     
- Misses       1143     1157      +14     
+ Partials      287      286       -1     

Files Changed	Coverage Δ
python/lsst/pipe/base/graphBuilder.py	61.53% <66.66%> (-1.78%)	⬇️

... and 1 file with indirect coverage changes

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[TallJimbo]

Thanks. With the benefit of hindsight this is much clearer.