Fix git-tree-sha1 for i686-w64-mingw32 GCC artifact

[topolarity]

Not sure how this was wrong, but saw this in CI and verified locally with ArtifactUtils.jl

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.16%. Comparing base (57505ca) to head (38447e9).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1056   +/-   ##
=======================================
  Coverage   86.16%   86.16%           
=======================================
  Files           3        3           
  Lines         795      795           
=======================================
  Hits          685      685           
  Misses        110      110           

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[KristofferC]

Thanks!