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.

test/runtime/Cargo.toml

@@ -35,7 +35,7 @@ cumulus-primitives-core = { path = "../../primitives/core", default-features = f
 cumulus-primitives-timestamp = { path = "../../primitives/timestamp", default-features = false }
 
 [build-dependencies]
-substrate-wasm-builder = { git = "https://github.com/paritytech/substrate", branch = "master" }
+substrate-wasm-builder = { git = "https://github.com/paritytech/substrate", branch = "master" , optional = true }
 
 [features]
 default = [ "std" ]
@@ -64,5 +64,6 @@ std = [
 	"cumulus-pallet-parachain-system/std",
 	"cumulus-primitives-core/std",
 	"cumulus-primitives-timestamp/std",
+	"substrate-wasm-builder",
 ]
 increment-spec-version = []

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() {}