Short-circuit when reading mill-version files

[fthomas]

If a .mill-version exists there is no need to also read .config/mill-version and then discard its content.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.77%. Comparing base (26f9dd0) to head (ead762a).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3541      +/-   ##
==========================================
- Coverage   89.77%   89.77%   -0.01%     
==========================================
  Files         171      171              
  Lines        4979     4977       -2     
  Branches      496      491       -5     
==========================================
- Hits         4470     4468       -2     
  Misses        509      509              

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