Add ability to optionally clean up wbuild/*/target in wasm-builder

[nazar-pc]

With switch to new Substrate version we started hitting space limits in GitHub Actions CI.
Turned out, wasm-opt-sys contributes a lot and generally with growing number of runtimes size of wbuild becomes a growing challenge.

It would be very helpful to be able to control whether wasm-builder can delete corresponding target after building WASM files (that are the only ones needed AFAK in the end). This would allow to use such a flag in CI because runtimes there are compiled once and never change.