BF+WIN: Protect against missing os.geteuid() on windows (fixes gh-2529)

[mih]

No description provided.

[codecov]

Codecov Report

Merging #2543 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2543      +/-   ##
==========================================
+ Coverage   90.56%   90.56%   +<.01%     
==========================================
  Files         240      240              
  Lines       29813    29808       -5     
==========================================
- Hits        27000    26996       -4     
+ Misses       2813     2812       -1

Impacted Files	Coverage Δ
datalad/interface/tests/test_unlock.py	92.77% <100%> (-0.26%)	⬇️
datalad/distribution/tests/test_clone.py	99.52% <100%> (-0.01%)	⬇️
datalad/support/annexrepo.py	89.75% <0%> (+0.07%)	⬆️

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