It looks like rustup run <foo> now only works when <foo> is one of rustc or cargo. I’ve been able to run multirust run nightly ./build-docs.sh with the old multirust when publishing new releases of rust-phf but that now fails:
$ rustup run nightly ./build_docs.sh
error: command failed: './build_docs.sh' (Io: No such file or directory (os error 2))