Fix charm refresh compatibility version tag during build

[carlcsaposs-canonical]

Currently, the charm refresh compatibility version gets marked as dirty since the working tree is not clean

This causes all refreshes to get marked as incompatible

Follow up to #866

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.35%. Comparing base (3f81441) to head (05d2df2).
Report is 7 commits behind head on 16/edge.

❌ Your project status has failed because the head coverage (69.35%) is below the target coverage (70.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           16/edge     #892      +/-   ##
===========================================
- Coverage    70.67%   69.35%   -1.33%     
===========================================
  Files           15       14       -1     
  Lines         3956     3955       -1     
  Branches       590      596       +6     
===========================================
- Hits          2796     2743      -53     
- Misses         970     1015      +45     
- Partials       190      197       +7     

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