crosslink: Fix Windows produces backslashes instead of slashes

[scorpionknifes]

Fixes: #458

Unit tests for crosslink now pass on Windows. (no changes in tests required)

opentelemetry-go-build-tools\crosslink>go test ./...
?       go.opentelemetry.io/build-tools/crosslink       [no test files]
ok      go.opentelemetry.io/build-tools/crosslink/cmd   0.085s
ok      go.opentelemetry.io/build-tools/crosslink/internal      0.204s

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (de501c5) 63.65% compared to head (3dd1561) 63.68%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #459      +/-   ##
==========================================
+ Coverage   63.65%   63.68%   +0.03%     
==========================================
  Files          39       39              
  Lines        2259     2261       +2     
==========================================
+ Hits         1438     1440       +2     
  Misses        677      677              
  Partials      144      144              

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