Set RUSTUP_TOOLCHAIN to the default version of the docker image

When a project uses the file `rust-toolchain.toml` or `rust-toolchain` (https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file), it is possible for the project to override the optimizer's Rust version. What will happen is that a different Rust version is downloaded and used during the build. However, the toolchain will be incomplete with missing target wasm32-unknown-unknown.

Now looking at the [order of toolchains used](https://rust-lang.github.io/rustup/overrides.html) we see that setting e.g. `export RUSTUP_TOOLCHAIN=1.72.0` overrides a project-specific setting. This ensure always the version of the optimizer is used, which is what we want.