[derive] Don't emit #[cfg(coverage_nightly)] (#2123)

As of nightly-2024-11-20 - specifically [1] - this triggers an
`unexpected_cfgs` lint even when emitted in derive-generated code.

[1] rust-lang/rust#132577

Fixes #2117

gherrit-pr-id: I9d536f0ea08475afe0b65411b225aa55f4db449a

Author: joshlf

zerocopy-derive/tests/issue_2117.rs

@@ -0,0 +1,20 @@
+// Copyright 2024 The Fuchsia Authors
+//
+// Licensed under a BSD-style license <LICENSE-BSD>, Apache License, Version 2.0
+// <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0>, or the MIT
+// license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option.
+// This file may not be copied, modified, or distributed except according to
+// those terms.
+
+// See comment in `include.rs` for why we disable the prelude.
+#![no_implicit_prelude]
+#![allow(warnings)]
+#![forbid(unexpected_cfgs)]
+
+include!("include.rs");
+
+// Make sure no unexpected `cfg`s are emitted by our derives (see #2117).
+
+#[derive(imp::KnownLayout)]
+#[repr(C)]
+pub struct Test(pub [u8; 32]);