use std::env::set_current_dir instead of libc::chdir

[japaric]

as per the API docs, the implementation uses chdir on unix systems.

[github-actions]

Number of dependencies and binary size impact report

Metric	main	PR #753	Delta
Direct dependencies	3	3	-
Total dependencies	4	4	-
Binary size	1 MiB	1 MiB	-
Text size	597.6 KiB	597 KiB	-

Dependencies diff

 └─ sudo-rs [v0.2.0]
    ├─ glob [v0.3.1]
    ├─ libc [v0.2.147]
    └─ log [v0.4.20]

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.03% 🎉

Comparison is base (0ac6fa8) 54.86% compared to head (1f2c635) 54.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #753      +/-   ##
==========================================
+ Coverage   54.86%   54.90%   +0.03%     
==========================================
  Files          71       71              
  Lines        9731     9724       -7     
==========================================
  Hits         5339     5339              
+ Misses       4392     4385       -7     

Files Changed	Coverage Δ
src/exec/mod.rs	0.00% <0.00%> (ø)
src/system/mod.rs	82.23% <ø> (+0.43%)	⬆️

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