Avoid the use of rustup

[webmaster128]

workspace-optimizer currently uses rustup to install Rust. This is a potential source of friction and in worst case an attack vector as rustup does not error on invalid Rust binary signatures (Switch warnings for errors, ensuring that GPG signatures are always checked and valid.).

If we find a way around the unstable --out-dir we can use the rust installation of the original docker image.

[maurolacy]

By using a shell script (or a shell script wrapper around optimize_workspace.py) I think we can easily avoid the use / need of --out-dir.

[webmaster128]

I'm sure it can be done. It was somewhat hard when I tried last time for various reasons. One is that when calling

docker run --rm -v "$(pwd)":/code \
  --mount type=volume,source="$(basename "$(pwd)")_cache",target=/code/target \
  --mount type=volume,source=registry_cache,target=/usr/local/cargo/registry \
  cosmwasm/workspace-optimizer:0.10.9

you will find target/**/*.wasm files from previous builds that are in the cosmwasm-plus_cache volume for some reason. You'd need to remove all of them before the build. If I recall correctly, there were other issues I ran into.