`permissions_set_readonly_false` false positive on Windows

Summary

permissions_set_readonly_false false positive when running Clippy on Windows.

Lint Name

permissions_set_readonly_false

Reproducer

I tried this code on Windows

let metadata = fs::metadata(path).map_err_context(String::new)?;
let mut perms = metadata.permissions();
#[cfg(not(unix))]
perms.set_readonly(false);

I saw this happen:

error: call to `set_readonly` with argument `false`
   --> src\uu\shred\src/shred.rs:475:9
    |
475 |         perms.set_readonly(false);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: on Unix platforms this results in the file being world writable
    = help: you can set the desired permissions using `PermissionsExt`. For more information, see
            https://doc.rust-lang.org/std/os/unix/fs/trait.PermissionsExt.html
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#permissions_set_readonly_false
    = note: `-D clippy::permissions-set-readonly-false` implied by `-D warnings`

I expected to see no error.

See https://github.com/uutils/coreutils/actions/runs/4383223472/jobs/7673213723

Version

rustc 1.68.0 (2c8cc3432 2023-03-06)
binary: rustc
commit-hash: 2c8cc343237b8f7d5a3c3703e3a87f2eb2c54a74
commit-date: 2023-03-06
host: x86_64-unknown-linux-gnu
release: 1.68.0
LLVM version: 15.0.6

Additional Labels

No response

Start with the permissions_set_readonly_false lint entry point and reproduce the diagnostic from the Windows-gated perms.set_readonly(false) call in src\uu\shred\src/shred.rs. Check the lint's platform handling and existing tests; done means this Windows case no longer produces a false positive while the Unix warning remains covered.