[builder] Close files immediately after writing is complete

[pjanotti]

Description:
Ensure that files written by cmd/builder are closed right after their content is written. This came about while investigating #5403

Link to tracking Issue:
#5403

Testing:
Local run on Windows

Documentation:
N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fa1b241) 90.44% compared to head (3f3cf9f) 90.42%.
Report is 6 commits behind head on main.

❗ Current head 3f3cf9f differs from pull request most recent head 2033c18. Consider uploading reports for the commit 2033c18 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9466      +/-   ##
==========================================
- Coverage   90.44%   90.42%   -0.02%     
==========================================
  Files         346      346              
  Lines       18106    18107       +1     
==========================================
- Hits        16376    16374       -2     
- Misses       1399     1401       +2     
- Partials      331      332       +1     

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