Generate fails in environment without rustup

[Toby222]

Running spacetime generate ... in an environment without rustup fails.
When trying to format the autogenerated files, it throws Error: No such file or directory (os error 2), complaining that the rustup binary is misisng here.
In my case, running NixOS, cargo and such are installed via nix.
A few lines above where it tries to install the wasm32-unknown-unknown target it correctly doesn't exit, and just tries to continue anyway.

Ideally it should only fail if rustfmt is missing, not if it fails to isntall rustfmt via rustup.
(Or formatting would simply be skipped on autogenerated files if errors occur, installing rustup components without the user's input does not seem good at all)