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/parachains/runtimes/assets/westmint/Cargo.toml

@@ -78,7 +78,7 @@ hex-literal = "0.3.4"
 asset-test-utils = { path = "../test-utils"}
 
 [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" ]
@@ -184,4 +184,5 @@ std = [
 	"parachain-info/std",
 	"parachains-common/std",
 	"assets-common/std",
+	"substrate-wasm-builder",
 ]