There were a number of issues:
1. `rustc` wasn't actually installed since the `INSTALL_RUST_VERSION`
argument wasn't set.
2. `rustup` wasn't running properly since the `CARGO_HOME` and
`RUSTUP_HOME`weren't being exported and set by default. We now use the
technique described in https://github.com/rust-lang/rustup/issues/1085
to create a wrapper and link all the binaries in /opt/rust/bin and
link them to /usr/local/bin.
3. `rustc version` is not a valid command. `rustc --version` is valid.