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/test/runtime/build.rs

@@ -14,9 +14,10 @@
 // You should have received a copy of the GNU General Public License
 // along with Cumulus.  If not, see <http://www.gnu.org/licenses/>.
 
-use substrate_wasm_builder::WasmBuilder;
-
+#[cfg(feature = "std")]
 fn main() {
+	use substrate_wasm_builder::WasmBuilder;
+
 	WasmBuilder::new()
 		.with_current_project()
 		.export_heap_base()
@@ -30,3 +31,6 @@ fn main() {
 		.set_file_name("wasm_binary_spec_version_incremented.rs")
 		.build();
 }
+
+#[cfg(not(feature = "std"))]
+fn main() {}