Make `--validate` aware of NOPASSWD

[squell]

Followup to #395: if a NOPASSWD rule applies to the user, original sudo doesn't ask for a password when running sudo -v. If we decide to copy this behaviour (requires some changes to the abstraction interface of sudoers) .

[BriocheBerlin]

Relevant test:
https://github.com/memorysafety/sudo-rs/blob/f5ef04c62f3b4279a3f79f2d078aedcdf3b70fa5/test-framework/sudo-compliance-tests/src/nopasswd.rs#L96

In combination with PASSWD:
https://github.com/memorysafety/sudo-rs/blob/f5ef04c62f3b4279a3f79f2d078aedcdf3b70fa5/test-framework/sudo-compliance-tests/src/nopasswd.rs#L111