fix: keep secure auth file mode warning-free on Windows

[ORESoftware]

Problem

The static mise certification matrix uncovered a Windows-only Clippy failure in current main:

src/auth.rs:899:21: unused variable: `path`

secure_file_mode uses its path only inside #[cfg(unix)], so Windows builds see an unused parameter and fail under cargo clippy -- -D warnings.

Fix

Rename the parameter to _path and use that name in the Unix-only permission branch. Runtime behavior is unchanged on every platform:

• Unix still applies mode 0600 to the same path.
• Non-Unix still performs no permission mutation and returns success.
• Windows Clippy no longer reports an unused variable.

Validation

This is intentionally a one-file, two-token semantic change. Existing repository workflows plus the Ubuntu/macOS/Windows static mise matrix remain the merge gates. Static mise PR #91 will stack this fix before its final immutable E2E pin.

Found by: zed-pkg-test/zed-pkg-e2e#7
Related static CLI: #91
Linear: DEN-1439, DEN-1449

[linear-code]

DEN-1605