removed +nightly command for cargo

[tanneberger]

Tiny pull request so the rust code is built with:

cargo build --release

instead of

cargo +nightly build --release

The problem is that the +nightly argument only exists in cargo from rustup. So standard the "standard" cargo will fail.

       error: no such subcommand: `+nightly`
       --> (unknown file):null:1 - cargo failed with error code 101 and reported the following error
       (error: no such subcommand: `+nightly`
       
       
       lfc: fatal error: Aborting due to previous error

=> the cargo in my env is nightly

[cmnrd]

Note that we already have #1218. Maybe you can sync with @jhaye on this matter ;)

[cmnrd]

Closing this since we merged #1218