Add tracking issue number to const_cstr_unchecked

Author: KamilaBorowska

library/std/src/ffi/c_str.rs

@@ -1257,7 +1257,7 @@ impl CStr {
     #[inline]
     #[must_use]
     #[stable(feature = "cstr_from_bytes", since = "1.10.0")]
-    #[rustc_const_unstable(feature = "const_cstr_unchecked", issue = "none")]
+    #[rustc_const_unstable(feature = "const_cstr_unchecked", issue = "90343")]
     pub const unsafe fn from_bytes_with_nul_unchecked(bytes: &[u8]) -> &CStr {
         // SAFETY: Casting to CStr is safe because its internal representation
         // is a [u8] too (safe only inside std).