Improve build times by disabling wasm-builder in no_std (#2308)

There is no need to compile wasm-builder in `no_std` as we skip the compilation of the crate any
way. So, we can also directly disable wasm-builder as dependency in `no_std` to improve build times.

cumulus/parachain-template/runtime/build.rs

@@ -1,9 +1,13 @@
-use substrate_wasm_builder::WasmBuilder;
-
+#[cfg(feature = "std")]
 fn main() {
-	WasmBuilder::new()
+	substrate_wasm_builder::WasmBuilder::new()
 		.with_current_project()
 		.export_heap_base()
 		.import_memory()
 		.build()
 }
+
+/// The wasm builder is deactivated when compiling
+/// this crate for wasm to speed up the compilation.
+#[cfg(not(feature = "std"))]
+fn main() {}