shells: add special nix behavior

[pedorich-n]

Following the discussion: #671 (comment), adding a special behavior for when rtx is installed under the /nix prefix: do not add rtx binary to the PATH environment variable.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -7.43 ⚠️

Comparison is base (6cc9e92) 88.08% compared to head (0126564) 80.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #675      +/-   ##
==========================================
- Coverage   88.08%   80.66%   -7.43%     
==========================================
  Files         129      129              
  Lines       11124    11159      +35     
==========================================
- Hits         9799     9001     -798     
- Misses       1325     2158     +833     

Impacted Files	Coverage Δ
src/shell/bash.rs	100.00% <100.00%> (ø)
src/shell/fish.rs	100.00% <100.00%> (ø)
src/shell/mod.rs	72.09% <100.00%> (-6.86%)	⬇️
src/shell/nushell.rs	100.00% <100.00%> (ø)
src/shell/xonsh.rs	100.00% <100.00%> (ø)
src/shell/zsh.rs	100.00% <100.00%> (ø)

... and 28 files with indirect coverage changes

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