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[bot]]

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

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

[codecov[bot]]

Codecov Report

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

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/753?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety) | Coverage Δ | | |---|---|---| | [src/exec/mod.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/753?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL2V4ZWMvbW9kLnJz) | `0.00% <0.00%> (ø)` | | | [src/system/mod.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/753?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3N5c3RlbS9tb2QucnM=) | `82.23% <ø> (+0.43%)` | :arrow_up: |

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