fix(settings): respect os in settings

[risu729]

MISE_OS or settings.os is added in #4224, but only test-tool respected it.

This PR replaces env::consts::OS or cfg!(target_os) with settings.os().

Since this will move the os detection from compile-time to runtime, it might slow down the execution.
Also, in most cases, setting MISE_OS doesn't work as expected.

This PR aims only to make the code consistent.
It might make OS-dependent tests a bit easier locally.

[github-actions]

If you are adding a new tool to mise, note the following:

• New asdf plugins are not accepted
• Tools may be rejected if they are not notable
• A test is required in registry.toml

[risu729]

We still need to replace #[cfg(windows)], but I doubt if this change is worth it.
Please feel free to close this PR.

Also, I think we should hide MISE_OS from the docs as it doesn't work well.