fix: use cfg to fix compiler warning for opts

Fixes #1621.

changelog.d/1620.misc

@@ -0,0 +1 @@
+Fixed compiler warning in cf_opts.rs when building in release. Contributed by @ezera.

src/database/engine/cf_opts.rs

@@ -70,17 +70,19 @@ fn descriptor_cf_options(
 		);
 	}
 
-	let mut opts = opts
+	let opts = opts
 		.get_options_from_string("{{arena_block_size=2097152;}}")
 		.map_err(map_err)?;
 
 	#[cfg(debug_assertions)]
-	let opts = opts
-		.get_options_from_string(
+	let opts = {
+		let mut opts = opts;
+		opts.get_options_from_string(
 			"{{paranoid_checks=true;paranoid_file_checks=true;force_consistency_checks=true;\
 			 verify_sst_unique_id_in_manifest=true;}}",
 		)
-		.map_err(map_err)?;
+		.map_err(map_err)?
+	};
 
 	Ok(opts)
 }