Only ignore on windows

Author: Jon Gjengset

tests/cli-rustup.rs

@@ -1516,7 +1516,10 @@ fn proxy_override_path() {
 }
 
 #[test]
-#[ignore = "FIXME: Windows uses UNC paths which do not work with relative paths"]
+#[cfg_attr(
+    windows,
+    ignore = "FIXME: Windows uses UNC paths which do not work with relative paths"
+)]
 fn file_override_path_relative() {
     setup(&|config| {
         expect_ok(config, &["rustup", "default", "stable"]);