DM-45726: Pass Butler to RepoExportContext rather than Registry

[timj]

Registry is being deprecated.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes
• (if changing dimensions.yaml) make a copy of dimensions.yaml in configs/old_dimensions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.44%. Comparing base (a0830e1) to head (aef9274).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1052      +/-   ##
==========================================
- Coverage   89.44%   89.44%   -0.01%     
==========================================
  Files         360      360              
  Lines       45844    45842       -2     
  Branches     9406     9406              
==========================================
- Hits        41006    41003       -3     
  Misses       3506     3506              
- Partials     1332     1333       +1     

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[andy-slac]

Looks great.