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.

parachains/runtimes/bridge-hubs/bridge-hub-polkadot/Cargo.toml

@@ -6,7 +6,7 @@ edition = "2021"
 description = "Polkadot's BridgeHub parachain runtime"
 
 [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 }
 
 [dependencies]
 codec = { package = "parity-scale-codec", version = "3.0.0", default-features = false, features = ["derive"] }
@@ -126,6 +126,7 @@ std = [
 	"xcm-builder/std",
 	"xcm-executor/std",
 	"xcm/std",
+	"substrate-wasm-builder",
 ]
 
 runtime-benchmarks = [

parachains/runtimes/bridge-hubs/bridge-hub-polkadot/build.rs

@@ -1,9 +1,26 @@
-use substrate_wasm_builder::WasmBuilder;
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
 
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#[cfg(feature = "std")]
 fn main() {
-	WasmBuilder::new()
+	substrate_wasm_builder::WasmBuilder::new()
 		.with_current_project()
 		.export_heap_base()
 		.import_memory()
 		.build()
 }
+
+#[cfg(not(feature = "std"))]
+fn main() {}