feat: add envs for rtx go core plugin

[dfang]

#677

for env names: RTX_GO_REPO RTX_GO_REPOSITORY RTX_GO_DOWNLOAD_MIRROR RTX_GOLANG_DOWNLOAD_MIRROR, since there is RTX_GO_SKIP_CHECKSUM already in src/env.rs, I choose short version too.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -6.44 ⚠️

Comparison is base (0145de9) 86.80% compared to head (aa43df9) 80.37%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #679      +/-   ##
==========================================
- Coverage   86.80%   80.37%   -6.44%     
==========================================
  Files         129      129              
  Lines       11159    11157       -2     
==========================================
- Hits         9687     8967     -720     
- Misses       1472     2190     +718     

Impacted Files	Coverage Δ
src/env.rs	84.75% <100.00%> (-3.74%)	⬇️
src/plugins/core/go.rs	78.39% <100.00%> (-10.30%)	⬇️

... and 18 files with indirect coverage changes

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[jdx]

I've gone ahead and run cargo fmt for you to fix the lint issues in this PR but please run that yourself on your other PRs

[dfang]

I've gone ahead and run cargo fmt for you to fix the lint issues in this PR but please run that yourself on your other PRs

ok, I will update code on local not GitHub web editor and run cargo fmt before commit.