message: update recommended CI snippet, add GHA example
diff: @@ -144,20 +144,37 @@ endian-sensitive code.
  ### Running Miri on CI
  To run Miri on CI, make sure that you handle the case where the latest nightly
  -does not ship the Miri component because it currently does not build. For
  -example, you can use the following snippet to always test with the latest
  -nightly that *does* come with Miri:
  +does not ship the Miri component because it currently does not build. `rustup
  +toolchain install --component` knows how to handle this situation, so the
  +following snippet should always work:
  ```sh
  -MIRI_NIGHTLY=nightly-$(curl -s https://rust-lang.github.io/rustup-components-history/x86_64-unknown-linux-gnu/miri)
  -echo "Installing latest nightly with Miri: $MIRI_NIGHTLY"
  -rustup set profile minimal
  -rustup override set "$MIRI_NIGHTLY"
  -rustup component add miri
  +rustup toolchain install nightly --component miri
  +rustup override set nightly
  cargo miri test
  ```
  +Here is an example job for GitHub Actions:
  +
  +```yaml
  + miri:
  + name: "Miri"
  + runs-on: ubuntu-latest
  + steps:
  + - uses: actions/checkout@v2
  + - name: Install Miri
  + run: |
  + rustup toolchain install nightly --component miri
  + rustup override set nightly
  + cargo miri setup
  + - name: Test with Miri
  + run: cargo miri test
  +```
  +
  +The explicit `cargo miri setup` helps to keep the output of the actual test step
  +clean.
  +
  ### Common Problems
  When using the above instructions, you may encounter a number of confusing compiler
  