Disable rustfix temporarily on extern-const.rs

`src/test/ui/extern/extern-const.rs` seems to have an inconsistent behaviour across build configurations, possibly non-deterministically. This is tracked in issue 54388.

For this PR, disable running rustfix on it because it failed on CI, until that issue is fixed.

src/test/ui/extern/extern-const.fixed

@@ -1,25 +0,0 @@
-// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-// run-rustfix
-// compile-flags: -Z continue-parse-after-error
-
-extern "C" {
-    static C: u8; //~ ERROR extern items cannot be `const`
-}
-
-fn main() {
-    // We suggest turning the (illegal) extern `const` into an extern `static`,
-    // but this also requires `unsafe` (a deny-by-default lint at comment time,
-    // future error; Issue #36247)
-    unsafe {
-        let _x = C;
-    }
-}

src/test/ui/extern/extern-const.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// run-rustfix
+// FIXME(#54388): re-enable rustfix later, when this test has consistent output across targets
 // compile-flags: -Z continue-parse-after-error
 
 extern "C" {