Change CI jobs to use stable toolchain of rust

[kushaldas]

Right now CI is failing because of using Rust nightly.

The following is a small reproducer:

use sequoia_openpgp as openpgp;
use openpgp::cert::prelude::*;
use sequoia_openpgp::parse::Parse;
use std::io;
use std::io::prelude::*;
use std::fs::File;

fn main() {
    let mut f = File::open("hellosecret.asc").unwrap();
    let mut buffer = Vec::new();
    f.read_to_end(&mut buffer).unwrap();

        let cert = openpgp::Cert::from_bytes(&buffer).unwrap();

    println!("Hello, world!");
}