parent_archs: Fix serializing of the value

[jan-kolarik]

Provide forgotten functionality to serialize parent_archs value into the manifest file. This way we should fetch only relevant noarch packages when working with multiarch manifests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.96%. Comparing base (3cc56d8) to head (99c06f3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #38      +/-   ##
==========================================
+ Coverage   80.79%   80.96%   +0.17%     
==========================================
  Files         154      154              
  Lines        2015     2033      +18     
==========================================
+ Hits         1628     1646      +18     
  Misses        387      387              

Flag	Coverage Δ
api	53.87% <ø> (ø)
impl	100.00% <100.00%> (ø)

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