Fix custom build.rs - deprecated build_current_project_with_rustflags

Look at latet version of `wasm_builder_runner` to fix this warning here:

https://github.com/Joystream/substrate-runtime-joystream/blob/385a376ed64e565ad410be76a7e424f0de340d66/build.rs#L26

```
warning: use of deprecated item 'substrate_wasm_builder_runner::build_current_project_with_rustflags': Please switch to [`WasmBuilder`]
```