Fix artifact name

[oliveromahony]

Proposed changes

Changed naming of artifact uploads from nginx-agent-snapshots to nginx-agent-unsigned-snapshots and nginx-agent-signed-snapshots

Checklist

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• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5efc8c2) 66.00% compared to head (8e82a38) 66.14%.
Report is 5 commits behind head on main.

❗ Current head 8e82a38 differs from pull request most recent head a584fd6. Consider uploading reports for the commit a584fd6 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #558      +/-   ##
==========================================
+ Coverage   66.00%   66.14%   +0.14%     
==========================================
  Files         118      118              
  Lines       13468    13461       -7     
==========================================
+ Hits         8889     8904      +15     
+ Misses       3979     3956      -23     
- Partials      600      601       +1     

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[olli-holmala]

Does this mean that there was a name collision?