Fix std.os.unlinkatW for absolute paths

RootDirectory can't be set in ObjectAttributes if ObjectName is an absolute path.

Fixes #4606

lib/std/os.zig

@@ -1438,7 +1438,7 @@ pub fn unlinkatW(dirfd: fd_t, sub_path_w: [*:0]const u16, flags: u32) UnlinkatEr
 
     var attr = w.OBJECT_ATTRIBUTES{
         .Length = @sizeOf(w.OBJECT_ATTRIBUTES),
-        .RootDirectory = dirfd,
+        .RootDirectory = if (std.fs.path.isAbsoluteWindowsW(sub_path_w)) null else dirfd,
         .Attributes = 0, // Note we do not use OBJ_CASE_INSENSITIVE here.
         .ObjectName = &nt_name,
         .SecurityDescriptor = null,