message: Switch Mac OS' CI to ARM.
  Hopefully installing the ARM toolchain will be faster and more reliable than installing the RISC-V toolchain. While I was editing the file, I changed he `checkout` action version to `v2`, so we should get automatic upgrades (I'm assuming it is relatively stable).
diff: name: ci-mac-os
  -# We run this workflow during pull request review, but not for Bors merges, as
  -# it takes over an hour to run.
  +# We run this workflow during pull request review, but not for Bors merges. We
  +# can change this if the workflow is reasonably quick and reliable.
  on: pull_request
  jobs:
  @@ -14,14 +14,21 @@ jobs:
  # Clones a single commit from the libtock-rs repository. The commit cloned
  # is a merge commit between the PR's target branch and the PR's source.
  - name: Clone repository
  - uses: actions/checkout@v2.3.0
  + uses: actions/checkout@v2
  +
  + # The brew commands are spread out over multiple steps so we can see the
  + # timings for each step in GitHub Actions' dashboard.
  + - name: Brew tap ARMmbed
  + run: brew tap ARMmbed/homebrew-formulae
  +
  + - name: Brew update
  + run: brew update
  +
  + - name: Brew install
  + run: brew install arm-none-eabi-gcc
  - # Install the toolchains we need, then run `cargo build`.
  - name: Build and Test
  run: |
  - brew tap riscv/riscv
  - brew update
  - brew install riscv-gnu-toolchain --with-multilib
  cd "${GITHUB_WORKSPACE}"
  - LIBTOCK_PLATFORM=hifive1 cargo build -p libtock_runtime \
  - --target=riscv32imac-unknown-none-elf
  + LIBTOCK_PLATFORM=nrf52 cargo build -p libtock_runtime \
  + --target=thumbv7em-none-eabi
  