Rust plugin broke in the last few days

I previously successfully built a snap which uses the rust plugin. https://github.com/popey/shellharden/tree/add-snapcraft - 3 days later it no longer builds. I have confirmed this on two computers, and a colleague has repeated the issue. I'm using snapcraft from stable with multipass.

error: Found argument '--prefix' which wasn't expected, or isn't valid in this context

USAGE:
    rustup-init [FLAGS] [OPTIONS]

For more information try --help
Failed to run '/root/parts/shellharden/rust/rustup.sh --prefix=/root/parts/shellharden/rust --disable-sudo --save --channel=stable' for 'shellharden': Exited with code 1.
Verify that the part is using the correct parameters and try again.
Run the same command again with --debug to shell into the environment if you wish to introspect this failure.
An error occurred when trying to execute 'sudo -i snapcraft snap' with 'multipass': returned exit code 2.