Only use substrate-wasm-builder when std feature is enabled. (#12790)

Co-authored-by: parity-processbot <>

substrate/bin/node-template/runtime/build.rs

@@ -1,9 +1,10 @@
-use substrate_wasm_builder::WasmBuilder;
-
 fn main() {
-	WasmBuilder::new()
-		.with_current_project()
-		.export_heap_base()
-		.import_memory()
-		.build()
+	#[cfg(feature = "std")]
+	{
+		substrate_wasm_builder::WasmBuilder::new()
+			.with_current_project()
+			.export_heap_base()
+			.import_memory()
+			.build();
+	}
 }