Remove duplicate toolchain build logic

[lrettig]

A lot of the logic in cli/src/commands/build_toolchain.rs duplicates logic found in https://github.com/athenavm/rustc-rv32e-toolchain/ clone.sh, build.sh, patch.sh. I left it intact for now since it's probably more platform independent to run some of these commands in Rust rather than using shell scripts, which might e.g. not work on Windows. We should sort this out and remove the duplicate logic.

I fixed one of these issues in #90 / athenavm/rustc-rv32e-toolchain#9 so at least we don't have to specify the Rust commit in two places.