PAM silent flag should not be set when a shell (-s) is requested

[rnijveld]

The silent flag was already not set for login shells, but this also disables setting it for normal shells, as the sudo manual specifies.

Fixes #489

[github-actions[bot]]

Number of dependencies and binary size impact report

Metric	main	PR #639	Delta
Direct dependencies	5	5	-
Total dependencies	10	10	-
Binary size	1.1 MiB	1.1 MiB	-
Text size	652.3 KiB	652.3 KiB	-

Dependencies diff

```diff └─ sudo-rs [v0.2.0-dev.20230703] ├─ glob [v0.3.1] ├─ libc [v0.2.147] ├─ log [v0.4.19] ├─ signal-hook [v0.3.15] | ├─ libc [v0.2.147] | ├─ signal-hook-registry [v1.4.1] | | └─ libc [v0.2.147] | └─ cc [v1.0.79] └─ signal-hook-registry [v1.4.1] ```

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 :tada:

Comparison is base (8ab1877) 86.84% compared to head (9193e63) 86.87%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #639 +/- ## ========================================== + Coverage 86.84% 86.87% +0.02% ========================================== Files 61 61 Lines 8644 8646 +2 ========================================== + Hits 7507 7511 +4 + Misses 1137 1135 -2 ``` | [Impacted Files](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/639?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety) | Coverage Δ | | |---|---|---| | [src/sudo/pam.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/639?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3N1ZG8vcGFtLnJz) | `97.08% <100.00%> (+0.05%)` | :arrow_up: | ... and [3 files with indirect coverage changes](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/639/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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