Fix bootstrap doctest failure

Author: nathanwhit

Diff for: library/std/src/ffi/c_str.rs

@@ -1266,7 +1266,7 @@ impl CStr {
     /// behavior when `ptr` is used inside the `unsafe` block:
     ///
     /// ```no_run
-    /// # #![allow(unused_must_use, temporary_cstring_as_ptr)]
+    /// # #![allow(unused_must_use)] #![cfg_attr(not(bootstrap), allow(temporary_cstring_as_ptr))]
     /// use std::ffi::CString;
     ///
     /// let ptr = CString::new("Hello").expect("CString::new failed").as_ptr();