Remove prerelease flag with uv in image builder

[thomasjpfan]

Why are the changes needed?

Using the prerelease flag will instead prerelease versions by default, which is unexpected. If users want to use prerelease builds they can set ImageSpec(env=dict(UV_PRERELEASE="allow")).

What changes were proposed in this pull request?

This PR removes the option.

How was this patch tested?

I ran image builder with this change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.63%. Comparing base (a8f68d7) to head (99f7fd2).
Report is 4 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2697       +/-   ##
===========================================
+ Coverage   78.91%   93.63%   +14.71%     
===========================================
  Files         316       78      -238     
  Lines       24965     3832    -21133     
  Branches     4012        0     -4012     
===========================================
- Hits        19702     3588    -16114     
+ Misses       4548      244     -4304     
+ Partials      715        0      -715     

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