Fix build_rust_wasm.sh script (correct shebang, don't require rustup)

[Philipp-M]

#!/bin/bash is non-standard, #!/usr/bin/env bash should be safer as it's standardized, as /bin/bash for instance doesn't exist on my system.

Additionally there are users (like me... this likely concerns a few NixOS users) who aren't using rustup, I've extended the script, such that it checks whether the rust target directory exists, in which case it's very likely that the required rust toolchain/target is on the system.
Similar for wasm-pack which might already be installed on the system without using cargo install.

[dmarcos]

Thank you!