su: make --pty the default

[japaric]

closes #634

[github-actions[bot]]

Number of dependencies and binary size impact report

Metric	main	PR #688	Delta
Direct dependencies	3	3	-
Total dependencies	4	4	-
Binary size	988 KiB	988 KiB	-
Text size	578.8 KiB	578.8 KiB	-

Dependencies diff

```diff └─ sudo-rs [v0.2.0-dev.20230711] ├─ glob [v0.3.1] ├─ libc [v0.2.147] └─ log [v0.4.19] ```

[codecov[bot]]

Codecov Report

Patch coverage: 66.66% and project coverage change: -0.07 :warning:

Comparison is base (6f8f1b5) 57.69% compared to head (1715357) 57.63%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #688 +/- ## ========================================== - Coverage 57.69% 57.63% -0.07% ========================================== Files 68 68 Lines 8971 8960 -11 ========================================== - Hits 5176 5164 -12 - Misses 3795 3796 +1 ``` | [Impacted Files](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/688?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety) | Coverage Δ | | |---|---|---| | [src/su/context.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/688?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3N1L2NvbnRleHQucnM=) | `61.84% <0.00%> (ø)` | | | [src/su/cli.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/688?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3N1L2NsaS5ycw==) | `92.78% <100.00%> (-0.26%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/688/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety)

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