fix env on directory change

[jdx]

Fixes #12

There are actually like 3 bugs that prevented this from working right. It wasn't overwriting env vars when the value changed (only new values), it also wasn't using the pristine environment from hook-env. This is a problem since the go plugin actually checks to see if GOPATH is already set, if so, it doesn't modify it. So we need to clear out GOPATH iff we set it previously.

[jdx]

Codecov Report

Base: 80.24% // Head: 81.73% // Increases project coverage by +1.48% 🎉

Coverage data is based on head (e358f0c) compared to base (bf15c1d).
Patch coverage: 75.00% of modified lines in pull request are covered.

❗ Current head e358f0c differs from pull request most recent head f592e73. Consider uploading reports for the commit f592e73 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
+ Coverage   80.24%   81.73%   +1.48%     
==========================================
  Files          59       59              
  Lines        1853     1872      +19     
==========================================
+ Hits         1487     1530      +43     
+ Misses        366      342      -24     

Impacted Files	Coverage Δ
src/cli/env.rs	100.00% <ø> (ø)
src/hook_env.rs	66.66% <46.15%> (-7.62%)	⬇️
src/env.rs	80.43% <60.00%> (+1.48%)	⬆️
src/cli/hook_env.rs	94.44% <100.00%> (+10.44%)	⬆️
src/config/config_file/tool_versions.rs	93.65% <100.00%> (+0.10%)	⬆️
src/env_diff.rs	100.00% <100.00%> (+2.22%)	⬆️
src/runtimes.rs	73.07% <100.00%> (+4.89%)	⬆️
src/shorthand_repository.rs	100.00% <0.00%> (ø)
src/config/mod.rs	84.51% <0.00%> (+1.93%)	⬆️
... and 3 more

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