Rustup not found causes publish to fail

[jdetter]

If you are trying to use spacetime publish or spacetime build and rustup isn't present on your system (e.g. you've installed cargo + rustc through your package manager) then it will fail with an error message:

[gefjon]

A correct solution to this issue should:

• First test if the wasm32-unknown-unknown target is installed. If it is, do nothing.
• If the target is not installed, check for rustup.
• If rustup is present, use it to install the target.
• If rustup is not present, present a message to the user asking them to either install rustup, or to install the wasm32-unknown-unknown target manually.