Remove workaround for lack of musl-dev now that musl-dev is provided in Rust Alpine images

Additionally, optimizes the build process a bit via leaving only the runtime
(and `busybox`) in the final image, and additionally building the runtime
without `std` (as we solely need the WASM blob from this process).

substrate/runtime/build.rs

@@ -1,4 +1,4 @@
 fn main() {
-  #[cfg(feature = "std")]
+  #[cfg(not(target_family = "wasm"))]
   substrate_wasm_builder::WasmBuilder::build_using_defaults();
 }