fix ~ expand cfg-gating of domainname for BSD-like platforms

[rivy]

Fixes #39.

[codecov]

Codecov Report

Merging #40 (3a9cea7) into main (cf75826) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
+ Coverage   94.60%   94.64%   +0.04%     
==========================================
  Files           4        4              
  Lines         926      933       +7     
==========================================
+ Hits          876      883       +7     
  Misses         50       50              

Flag	Coverage Δ
macos_latest	99.31% <100.00%> (+0.03%)	⬆️
ubuntu_latest	99.31% <100.00%> (+0.03%)	⬆️
windows_latest	94.11% <ø> (ø)

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Impacted Files	Coverage Δ
src/platform/unix.rs	99.00% <100.00%> (+0.07%)	⬆️

[sylvestre]

i pushed a 2.0.1 with this fix to unblock coreutils :)
thanks!