Change modifiers S3 artifact metadata to be a list of features

[NotTheEvilOne]

What this PR does / why we need it:
This PR fixes an regression in the generated S3 artifact metadata. modifiers should be a list of strings but was formatted as a ,-separated string instead.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.65%. Comparing base (ab55b23) to head (febc8c9).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gardenlinux/features/cname.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #312      +/-   ##
==========================================
- Coverage   91.68%   91.65%   -0.03%     
==========================================
  Files          42       42              
  Lines        2129     2134       +5     
==========================================
+ Hits         1952     1956       +4     
- Misses        177      178       +1     

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[saubhikdattagithub]

LGTM, as discussed