fix(mutate): close layer readers immediately after extracting layer

[caxu-rh]

In mutate.extract, the closing of layerReader resources is deferred from inside a for loop. As a result, the layerReaders are left open until all layers are processed, which may accumulate on images with many layers.

Intorduce an inner function/closure to contain the layerReader and operations that use it, so that it can be released immediately after the current layer has been processed.

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[caxu-rh]

Sorry for the delay - resolved the CLA coverage. The diff looks kind of confusing, but this is essentially nesting the loop body into an anonymous function.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.37838% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.93%. Comparing base (8b3c303) to head (a122583).
⚠️ Report is 146 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/v1/mutate/mutate.go	78.37%	4 Missing and 4 partials ⚠️

❗ There is a different number of reports uploaded between BASE (8b3c303) and HEAD (a122583). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (8b3c303)	HEAD (a122583)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2190       +/-   ##
===========================================
- Coverage   71.67%   52.93%   -18.75%     
===========================================
  Files         123      165       +42     
  Lines        9935    11211     +1276     
===========================================
- Hits         7121     5934     -1187     
- Misses       2115     4562     +2447     
- Partials      699      715       +16     

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