feat: use proxy from environment

[caarlos0]

closes #1879

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[codecov]

Codecov Report

Merging #1885 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1885      +/-   ##
==========================================
+ Coverage   81.53%   81.55%   +0.01%     
==========================================
  Files          73       73              
  Lines        3747     3751       +4     
==========================================
+ Hits         3055     3059       +4     
  Misses        570      570              
  Partials      122      122              

Impacted Files	Coverage Δ
internal/client/gitea.go	90.35% <100.00%> (+0.08%)	⬆️
internal/client/github.go	24.16% <100.00%> (+0.51%)	⬆️
internal/client/gitlab.go	12.01% <100.00%> (+0.37%)	⬆️
internal/http/http.go	89.08% <100.00%> (+0.06%)	⬆️

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[github-actions]

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