Add unix only tag for unix-only import in LSP test (#1081)

Summary:
This diff fixes open source windows build.

In L78-L83 of the file, the code block accessing unix permissions are already tagged `#[cfg(unix)]`. This diff adds the same config tag on the import so that windows build can be fixed.

Pull Request resolved: #1081

Reviewed By: kinto0, ndmitchell

Differential Revision: D82327050

Pulled By: rchen152

fbshipit-source-id: 7bc4cd173ca9c0a068ff2768708416cb7525fbae

Author: fangyi-zhou

pyrefly/lib/test/lsp/lsp_interaction/configuration.rs

@@ -6,6 +6,7 @@
  */
 
 use std::fs;
+#[cfg(unix)]
 use std::os::unix::fs::PermissionsExt;
 
 use lsp_server::Message;