feat: support for PATH in .rtx.toml env vars

[jdx]

This makes it so you can add new directories to PATH in .rtx.toml. This is done by specifying PATH as an array like the following:

[env]
PATH = [
  "/foo",  # absolute path
  "./bar", # relative to the directory .rtx.toml is in
  "$PATH" # for now this is required to be at the end
]

[codecov]

Codecov Report

Patch coverage: 93.24% and project coverage change: +0.02 🎉

Comparison is base (a74ff74) 88.32% compared to head (cea77f2) 88.34%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #313      +/-   ##
==========================================
+ Coverage   88.32%   88.34%   +0.02%     
==========================================
  Files         100      100              
  Lines        8074     8145      +71     
==========================================
+ Hits         7131     7196      +65     
- Misses        943      949       +6     

Impacted Files	Coverage Δ
src/config/config_file/mod.rs	94.00% <ø> (ø)
src/config/config_file/rtx_toml.rs	84.42% <91.52%> (+0.78%)	⬆️
src/config/config_file/legacy_version.rs	56.14% <100.00%> (+2.43%)	⬆️
src/config/config_file/tool_versions.rs	99.41% <100.00%> (+0.01%)	⬆️
src/config/mod.rs	84.43% <100.00%> (+0.32%)	⬆️
src/toolset/mod.rs	75.46% <100.00%> (ø)

... and 1 file with indirect coverage changes

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