Fix: Publish fails if rustup not present

[bfops]

Description of Changes

Allow users to run spacetime publish even if rustup is not present on their system. Typically we use it to install the wasm32-unknown-unknown target.

Addresses #799

API and ABI breaking changes

These are CLI-only changes that don't affect our ABI.

Expected complexity level and risk

1

[bfops]

Fixes #799

[jdetter]

Small nit, but looks good to me 👍

[bfops]

Oops I forgot to include this, but my testing was:

$ mv ~/.cargo/bin/rustup{,-backup}
$ cd $WORK/my_test_module
$ cargo run publish -c -S publish-test
Warning: Failed to install wasm32-unknown-unknown target: No such file or directory (os error 2). Is `rustup` installed?
    Finished release [optimized] target(s) in 0.05s
Optimising module with wasm-opt...
Could not find wasm-opt to optimise the module.
...

and then afterward to restore rustup:

$ mv ~/.cargo/bin/rustup{-backup,}