Unable to build stub for RISC-V targets using xtask after switching to stable toolchain

[jessebraham]

In #70 we switched from nightly to stable, however unfortunately the CI was not correctly updated and is still performing checks using nightly.

As a result, the stubs are currently not buildable when using the xtask package and targeting RISC-V devices due to the inclusion of the -Z flag, which is unstable, e.g.)

jesse@mbp ~/W/e/xtask git:fixes/update [ ✎ ⚑ ] 
🦀 [email protected] λ 
cargo run -- build esp32c2
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.06s
     Running `target/debug/xtask build esp32c2`
error: the `-Z` flag is only accepted on the nightly channel of Cargo, but this is the `stable` channel
See https://doc.rust-lang.org/book/appendix-07-nightly-rust.html for more information about Rust release channels.

cc @SergioGasquez @JurajSadel