Install xargo with miri toolchain

[dtolnay]

This speeds up any subsequent miri invocation by 1m 37s. Previously they used to compile xargo from source on first run of cargo miri.

Running `"/home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo" "install" "xargo"` to install a recent enough xargo.
    Updating crates.io index
 Downloading crates ...
  Downloaded xargo v0.3.26
  Installing xargo v0.3.26
.....
    Finished release [optimized] target(s) in 1m 37s
  Installing /home/runner/.cargo/bin/xargo
  Installing /home/runner/.cargo/bin/xargo-check
   Installed package `xargo v0.3.26` (executables `xargo`, `xargo-check`)

dtolnay/semver#283