Fix build warning on #[ignore] attrbute

The full warning:

    warning: attribute must be of the form `#[ignore]` or `#[ignore = "reason"]`
    = note: #[warn(ill_formed_attribute_input)] on by default
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #57571 <rust-lang/rust#57571>

tests/cli-rustup.rs

@@ -686,7 +686,7 @@ fn list_default_toolchain() {
 }
 
 #[test]
-#[ignore(windows)] // FIXME Windows shows UNC paths
+#[ignore = "FIXME: Windows shows UNC paths"]
 fn show_toolchain_override() {
     setup(&|config| {
         let cwd = config.current_dir();
@@ -709,7 +709,7 @@ nightly-{0} (directory override for '{1}')
 }
 
 #[test]
-#[ignore(windows)] // FIXME Windows shows UNC paths
+#[ignore = "FIXME: Windows shows UNC paths"]
 fn show_toolchain_toolchain_file_override() {
     setup(&|config| {
         expect_ok(config, &["rustup", "default", "stable"]);
@@ -748,7 +748,7 @@ nightly-{0} (overridden by '{1}')
 }
 
 #[test]
-#[ignore(windows)] // FIXME Windows shows UNC paths
+#[ignore = "FIXME: Windows shows UNC paths"]
 fn show_toolchain_version_nested_file_override() {
     setup(&|config| {
         expect_ok(config, &["rustup", "default", "stable"]);
@@ -792,7 +792,7 @@ nightly-{0} (overridden by '{1}')
 }
 
 #[test]
-#[ignore(windows)] // FIXME Windows shows UNC paths
+#[ignore = "FIXME: Windows shows UNC paths"]
 fn show_toolchain_toolchain_file_override_not_installed() {
     setup(&|config| {
         expect_ok(config, &["rustup", "default", "stable"]);