Rust templates fail to build/missing RISC-V target architecture

[masiedu4]

Ran into an issue while trying to build the Rust template.

• cartesi build works fine (because Dockerfile handles RISC-V setup)
• But cargo build locally fails with:these errors:

error[E0463]: can't find crate for `core`
  |
  = note: the `riscv64gc-unknown-linux-gnu` target may not be installed
  = help: consider downloading the target with `rustup target add riscv64gc-unknown-linux-gnu`

error[E0463]: can't find crate for `std`

Fix

To provide value for developers who want to use their IDE's Rust support

1. Add the RISC-V target

rustup target add riscv64gc-unknown-linux-gnu

2. Edit the Cargo.toml file

[build]
target = "riscv64gc-unknown-linux-gnu" 

or build with(on Apple SIlicon)

cargo build --target aarch64-apple-darwin

Docs

Could we add a small note in here about this? Something like:

If you're building the Rust template, make sure to run rustup target add riscv64gc-unknown-linux-gnu first.
Otherwise, you'll get compilation errors about missing core and std crates.